
Class 
Book. 



_ 



GcBrigfttf. 



C9H£RJGHT DEPOS1H 

7 



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AMERICAN 



TELEPHONE PRACTICE "7 



BY 

KEMPSTER B. MILLER 



FOURTH EDITION, ENLARGED AND ENTIRELY REWRITTEN 



NEW YORK 

McGRAW PUBLISHING COMPANY 

114 Liberty Street 
1905 




LIBRARY of CONGRESS 
jwt Copies rioctJtveo 

APR 7 iyu5 

Gopyngiu tniry 

CLASS ou XXc Noj 

COPY B. 






Copyrighted, 1900, 

by the 

AMERICAN ELECTRICIAN COMPANY, 

and 

Copyrighted, 1905, 

• by the 

McGRAW PUBLISHING COMPANY, 

New York. 



TO THE MEMORY OF MY FATHER, JOSEPH KEMPSTER 
MILLER, THIS BOOK IS DEDICATED. IT WAS THROUGH 
HIS INFLUENCE THAT THE WORK WAS BEGUN. 

Kempster B. Miller. 



PREFACE TO FIRST EDITION. 



The intended scope of this book is set forth in its title. To 
those interested the writer has endeavored to present in as clear 
a manner as possible the general principles of telephony, the design 
and construction of commercial apparatus, the circuits connecting 
such apparatus into operative systems, and the methods used in 
the construction, operation and maintenance of these systems. No 
attempt whatever has been made to treat the subject -from its purely 
mathematical standpoint, that being beyond the scope of this work. 
The apparatus and methods of both Bell and independent companies 
have been given impartial attention. 

The writer sincerely thanks his friends, Mr. Wm. H. Donner and 
Mr. Wm. R. Mackrille, for their many suggestions and untiring 
labors in proof-reading, and also Mr. W. D. Weaver, editor of the 
Electrical World and Engineer, for his interest and assistance 
throughout the entire preparation of this book. 

Kempster B. Miller. 



PREFACE TO THIRD EDITION, 



That a third edition of this work should have been called for 
within three months after the appearance of the first is indeed grati- 
fying. The time since the second edition was exhausted has been 
utilized in making many changes in the matter already presented, 
and in the preparation of much new matter, all of which it is thought 
will make the work more valuable as a guide and general reference- 
book in practical telephony. 

The chapter on Automatic Exchanges has been written because 
the book seemed incomplete without it. If it is hereafter criticised 
for containing no descriptions of practical apparatus, my plea will 
be that it is mainly the fault of the subject. 

That very important factor in modern telephony, the t storage 
battery, was certainly very inadequately handled in the first edition. 
Chapter XXXV. is intended to remedy this. 

The last chapter on Specifications is given as a help rather than 
as an inflexible guide to those drawing specifications. The specifi- 
cations given represent modern practice, but they are not meant to 
serve in the place of common sense or of engineering ability. 

I desire to thank Mr. Thomas D. Lockwood for his kindly criti- 
cisms on the first editions ; Mr. Franz J. Dommerque, who has read 
the proofs of the chapters on Storage Batteries and Specifications, 
and made many good suggestions ; and Mr. E. R. Corwin, to whom 
I am indebted for information concerning lead-burning. 

Kempster B. Miller. 



VI 



PREFACE TO FOURTH EDITION. 



The revision for this edition has resulted in an almost complete 
rewriting, upon which far more labor has been expended than in 
the original writing of the early editions. During the past year and 
a half in which the work has been done, the magnitude of the in- 
tended task has steadily grown, owing to the development of the 
telephone field, and to the wider view of this field made possible by 
my own changed environment. 

The work is now believed to cover telephone practice of to-day 
much more comprehensively and accurately than was true of any 
of the earlier editions with respect to the practiced art at the times 
of their publication. Obsolete methods and equipment are not dealt 
with, except where of distinct educational or historic value ; and 
much of the historic matter of former editions has been cut out in 
order to make room for modern methods and things. 

The classification according to chapters has been materially 
altered, many of the old chapters having been combined with others 
or subdivided, in order to afford a more logical arrangement. The 
chapter on The Telephone Exchange in General has been added as 
an introduction to the subsequent chapters, which deal with the 
various parts of the exchange, particularly with the switch-board 
systems, appliances and auxiliaries. 

When former editions were written, the common battery or central 
energy systems were comparatively new and little information con- 
cerning them was available. The resulting lack of full information 
on this subject, for which the early editions might have been criti- 
cised with justice, is now thought to have been removed. 

Chapters XVI. to XXXV., inclusive, are almost entirely new. 
although they contain some matter that was barely touched upon in 
previous editions. Such subjects as trunking between common 
battery offices, private branch exchange service, measured service. 
toll switch-board systems and power plants are now here treated for 
the first time. 

There has been rapid recent development in automatic switch- 
boards and the complete description of the modern Strowger system, 

vii 



vin PREFACE TO FOURTH EDITIOX. 

contained in Chapter XXXY., is indicative of the increasing impor- 
tance of this branch. 

This work is intended not only as a guide to the student of prac- 
tical telephony whose experience has not been sufficient to make 
him conversant with all branches of the subject, but also as an aid 
to the more experienced telephone engineer and operator, who may 
find it of value as a general reference work. 

To my partner, Mr. Samuel G. McMeen, and to my former asso- 
ciate, Mr. Charles S. Winston, now Chief Engineer of the Kellogg 
Switchboard and Supply Company, are due my first acknowledge- 
ments for valuable assistance. I have drawn freely on their funds 
of knowledge, judgment and good nature, and on their much more 
limited funds of time. 

I feel some pride in the illustrations, most of which are quite new. 
This is true particularly of the diagrams of complicated circuits, 
which are thought to be more complete than any thing heretofore 
attempted in this line. Much of the credit for the appearance and 
arrangement of these is due to the skill of Mr. Howard M. Post, 
who drew most of them. To Mr. R. H. Burfiend credit is given for 
the appearance of many of the apparatus drawings and some of the 
circuit work. 

The various telephone manufacturing companies have co-operated 
by loaning photographs and half-tones illustrative of modern ap- 
paratus and work. Especially is this true of the Stromberg-Carlson 
Telephone Manufacturing Company, the Kellogg Switchboard and 
Supply Company, the Holtzer Cabot Electric Company, and the 
Automatic Electric Company. 

In the preparation of the manuscript for a work of this nature 
the amanuensis may be a boon or a "calamity." I tender sincere 
thanks to Miss B. R. Werden and Miss Eva A. Garlock for their 
unusually painstaking and intelligent work in preparing all the man- 
uscript from my dictation and notes, and in the proof-reading. 

I feel that the public will, as the writer does, thank the makers of 
this book, the McGraw Publishing Company, for the manner in 
which they have done their work. Their patience is also to be com- 
mended. 

The kindly reception of the earlier editions is keenly appreciated, 
and it is my hope that the work is now more nearly worthy. 

Kempster B. Miller. 



TABLE OF CONTENTS. 



CHAPTER I 

Wtcthdv a atti T , r>TTvrr , Tt>T ire r\v i-v 



PAGE 

History and Principles of the Magneto Telephone, i 



Early Knowledge of Electromagnetism — Work of Oersted, Ampere, 
Arago and Davy, Sturgeon, Faraday and Henry — Transformation 
of Electric into Magnetic Energy — Transformation of Magnetic 
into Electric Energy — Field of Force — Morse's Telegraph — Reis' 
Telephone — Sound Waves — Bell's Telephone — House's Electro- 
Phonetic Telegraph. 

CHAPTER II. 
History and Principles of the Variable Resistance Transmitter, . 13 

Gray's Variable Resistance Transmitter — Bell's Liquid Transmitter — 
Berliner's Transmitter — Electrodes in Constant Contact — Carbon 
Electrodes — Demonstration of Advantages of Loose Contact by 
Hughes — Hughes' Microphone — Hunning's Granular Carbon Trans- 
mitter — Induction Coil with Transmitter. 

CHAPTER III. 
Electromagnetic and Electrostatic Induction, .... 23 

Ohm's Law — Field of Force about Conductor — Electromagnetic Induc- 
tion — Action between Turns of the Same Coil — Impedance — Effects 
of Self-induction on Undulatory Currents — Charge of Electricity — 
Action between Like and Unlike Charges — Electrostatic Induction — 
Condensers — Capacity — Specific Inductive Capacity of Dielectrics — 
Specific Inductive Capacity in Telephone Cables — Effect of Condenser 
Bridged across Circuit — Effect of Capacity on Carrying Currents — 
Trans-Oceanic Telephony. 

CHAPTER IV. 
The Telephone Receiver, 34 

Considerations in Designing — Mechanical and Electrical Efficiency — 
Single-Pole Receivers — Bipolar Receivers — Adjustment between 
Magnet and Diaphragm — Material for Shells — Faults of Imitation 
Hard Rubber — Commercial Types oi Receivers — Receiver Cords — 
Details of Cord-Tip — Supports for Receiver Cords. 



PAGE 



x TABLE OF COXTEXTS. 

CHAPTER V. 
The Carbox Transmitter, ......... 53 

Action of the Transmitter — Single-Contact Transmitters — Multiple-Con- 
tact Transmitters — Granular Carbon Transmitters — Commercial 
Types of Transmitters — Packing : Its Remedy — Unusual Forms of 
Transmitters. 

CHAPTER VI. 

Induction Coils for Local Battery Telephones, . . . -73 

Advantages of the Induction Coil — Primary Current — Secondary Cur- 
rent — Design of Induction Coils — Commercial Coils — Varley Method 
of Winding — Mounting of Induction Coils — Results of Comparative 
Tests — Methods of Making Comparative Tests. 

CHAPTER VII. 
Primary Batteries-, 85 

Simple Cell — Direction of Current — Positive and Negative Poles — Ma- 
terials Best Suited for Electrodes — The LeClanche Cell — The Fuller 
Cell — Specification for Standard Fuller Cell — The Gravity Cell — The 
Gordon Cell — The Dry Cell — Comparative Battery Tests. 

CHAPTER VIII. 
Magneto Calling Apparatus, . . . . . . . .104 

Battery Calls — Magneto Generator — Its Action — The Magneto Bell — 
Factors Governing the Output of Generators — Design of Generators 
— Wave-Form of Magneto Generators — Points in Design of Mag- 
neto Bell — Varley Windings — The Manually Operated Shunt — The 
Automatic Shunt — Commercial Forms of Generators and Ringers — 
The Biased Bell. 

CHAPTER IX. 

Local Battery Sub-Station Equipments, 131 

The Term Sub-Station — Series Equipments — Bridging Equipments — The 
Hand-Switch — The Automatic Hook-Switch — Circuits of Series 
Sub-Station Equipment — Types of Bridging Circuits — Complete 
Telephones — Circuits of Complete Telephone — Portable Desk 
Stands — Types of Desk Stand Circuits — Commercial Types of 
Switch Hooks. 

CHAPTER X. 
Telephone Lines, 158 

The Grounded Line — Work of J. J. Carty — Electromagnetic Disturbances 
on Telephone Lines — Electrostatic Disturbances — Transposition of 
Telephone Lines — The Repeating Coil — The Uses of the Repeating 
. Coil. 



TABLE OF CONTENTS. xi 

CHAPTER XI. 

PAGE 

The Telephone Exchange in General, 170 

The Functions of the Telephone Exchange — Definition of Telephone 
Office — Definition of Telephone Exchange — Single-Office Exchanges 
— Two-Office Exchanges — Multi-Office Exchanges — Manually Oper- 
ated Exchanges — Automatic Exchanges — Early Idea of Central 
Office Exchange Work — The Law System — The Prototype of the 
Modern Switch-Board. 



CHAPTER XII. 
The Magneto Switch-Board for Small Exchanges, .... 176 

Simplest Switch-Board for Grounded Lines — The Spring Jack — The 
Switch-Board Drop — Details of Circuits for Simple Switch-Boards — 
The Metallic Circuit Jack — The Plug and Cord — Types of Com- 
mercial Drops and Jacks — Ringing and Listening Keys — Cord Circuits 
— Self-Restoring Drops — Combined Drops and Jacks — Complete 
Magneto Switch-Boards — Audible Line Signals — Toll Cut-in Sta- 
tions — The Wiring of Switch-Boards. 

- CHAPTER XIII. 
The Theory of the Multiple Switch-Board, 219 

Necessity for Multiple Boards — Fundamental Object of Multiple Boards 
— Answering Jacks — Multiple Jacks — Sections of Switch-Board — 
Operators' Positions — The Busy Test — Limiting Factors in Switch- 
Board Capacity — End Positions. 

CHAPTER XIV. 
The Magneto Multiple Switch-Board, 225 

Switch-Board for Grounded Lines — The Series Multiple Board — The 
Courtlandt Street Switch-Board in New York — The Branch Ter- 
minal Multiple Board — Spring Jacks for Multiple Boards — The 
Paris Switch-Board. 

CHAPTER XV. 
Transfer Systems, 245 

Apparent Desirability of Transfer Systems — Saving in Jacks — The Sabin 
& Hampton Express System — A Boards — B Boards — Arrangement of 
Circuits in the Sabin & Hampton System — Automatic Calling — Auto- 
matic Clearing-Out — The Multiple Plug Transfer System — Switch- 
Board at Wilmington, Delaware — Old Transfer System at Grand 
Rapids, Michigan — Transfer System for Small Switch-Board. 



xii TABLE OF CONTENTS. 

CHAPTER XVI. 

PAGE 

Systems of Transmission in Common Battery Exchanges, . . 205 
Advantages of Locating Sources of Energy at the Central Office — Various 
Methods of Supplying Transmitter Current from Central Office — The 
Stone System — The Hayes System — Kellogg Two-Battery System — 
The Dean System — Electrolytic Cells at Subscribers' Stations — Ther- 
mopile at Subscribers' Stations. _ 

CHAPTER XVII. 

Signaling in Common Battery Systems, 277 

Fundamental Requirements in Automatic Signaling — Difference between 
Common Battery and Magneto Signaling — Mechanical Signals — 
Lamp Signals — Elements of Sub-Station Circuits — Signaling without 
Relay — Signaling with Relay — The Cut-off Jack — The Cut-off Relay 
— Supervisory Signals — Various Methods of Controlling Supervisory 
Signals — Types of Switch-Board Lamps — Types of Mechanical Sig- 
nals. 

CHAPTER XVIII. 
Common Battery Switch-Boards in Small Exchanges, . . . 293 
Typical Circuits — Operation of Complete System — System Using Direct 
Lamps — Cabinet for Small Switch-Boards. 

CHAPTER XIX. 

Common Battery Sub-Station Equipment, 304 

Simple Sub-Station Circuit — Circuit with Condenser — Western Electric 
Sub-Station Circuit — Stromberg-Carlson Sub-Station Circuit — Kel- 
logg Sub-Station Circuit — Common Battery Telephone Sets. 

CHAPTER XX. 
The Common Battery Multiple Switch-Board, .... 313 
Western Electric System — Details of Operation of System — Wiring Cir- 
cuit of Line and Pilot Signals — The Kellogg Two-Wire System — 
Three-Wire vs. Two-Wire System — The Stromberg-Carlson Two- 
Wire System — The Stromberg-Carlson Three-Wire System — The 
Sterling Multiple Board — The North System — The International 
System — The American Electric Telephone Company's System — The 
Bell Switch-Board at St. Louis. 

CHAPTER XXI. 
Trunking System Between Common Battery Offices, . . . 354 
Necessity of Trunking — Outgoing and Incoming Trunk Lines — A and 
B Positions — Approved Method of Trunking — Western Electric 
Trunk Circuit — Its Operation — Automatic Ringing — The Kellogg 
Trunk Circuit — The Stromberg-Carlson Trunk Circuit — Guarding 
Against Mistakes in the Operation of Trunk Circuits. 



TABLE OF CONTENTS. xiii 

CHAPTER XXII. 

PAGE 

The Divided Multiple System, 378 

Limitation of Ordinary Multiple Switch-Board — A Method of Increasing 
the Possible Size of Multiple Switch-Boards — Capacity of the Di- 
vided Multiple System — Schematic Treatment of Divided Multiple 
,, System — Method of Signaling — The System 'at St. Louis and Cleve- 
land — The Divided Multiple for Common Battery — The Four-Divi- 
sion Board at Cleveland, Ohio. 

CHAPTER XXIII. 

Private Branch Exchange Service, ....... 396 

Definition of Private Branch Exchange — Local Connection — Single 
Track vs. Double Track System — Division of Work between Opera- 
tors — Methods of Operation in Private Branch Exchanges — Private 
Branch Exchange Circuits — Complete Private Branch Exchange 
Switch-Boards. 

CHAPTER XXIV. 
Party Line Systems, 423 

General Classification of Party Lines — The Carty Bridging Bell System 
— Early Types of Party-Line Signaling — The Hibbard System — The 
McBerty System Using a Jack for Each Station — The Thompson 
& Robes System — The Dean Harmonic System — The Leich System. 

CHAPTER XXV. 
Measured Service, . 460 

Methods of Charging for Telephone Service — "Deadhead" Business — 
The Measuring of Telephone Service — Coin-Collecting Devices — 
The Baird Pay-Station — The Gray Station — The Scribner Coin-Col- 
lecting Device — The Stroud Coin-Collecting Device — Circuits of 
Various Coin-Collecting Systems — Telephone Meters or Counters — 
Counters at the Subscribers' Stations — Counters at the Central Office 
— The Western Electric Counter or Meter. 



CHAPTER XXVI. 

Toll Switch-Board Systems, 48- 

Points in the Operation of Toll-Boards — Desirability of giving Regular 
Operators no Work of Special Nature — Toll-Line' Operators — Toll 
Recording Operators — Three Methods of Handling Toll Service — 
Toll-Board Sections. 



xiv TABLE OF COXTEXTS. 

CHAPTER XXVII. 

PAGH 

Details of Multiple Switch-Board Apparatus, .... 513 

Three-Wire Jacks — Two-Wire Jacks — Grouping of Jacks in Strips or 
Banks — Numbering of Multiple Jacks — Lamp Jacks — Arrangement 
of Answering and Multiple Jacks — Details of Switch-Board Frame — 
Lamp Mounting — Cords and Plugs — Cord Weights — Ringing and 
Listening Keys — Order Wire Keys — Cut-in Jack and Plug — Opera- 
tors' Transmitting Mounting — Types of Relays. 

CHAPTER XXVIII. 
Power Plants ix Common Battery Systems, 544 

Old Types of Power Plants — The Necessity of Reliable Primary Power 
— The Circuits of a Modern Power System — Power Switch-Boards — 
Methods of Wiring Power Switch-Boards — Impedance Coils in 
Power Plants — Magneto Generators — Various Types of Charging 
and Ringing Machines — Busy-Back Attachments — Power Tables — 
Rheostats. 

CHAPTER XXIX. 
Storage Batteries, 572 

Simple Storage Cell — The Plante Cell — The Chloride Accumulator — 
Glass and Wooden Tanks — The American Battery — Lead Burning — 
The Electrolyte — The Charging of Batteries — Determination of 
Amount of Charge — Replacing of Electrolyte — Treatment of Injured 
Cells — Color of Plates — Taking Batteries out of Service. 

CHAPTER XXX. 
Protective Devices 58S7 

Problems Involved in Adequate Protection — Elements Against which 
Protection Must Be Made — The Saw-Tooth Arrester — The Carbon 
Block Arrester — Fusible Cut-Outs — The Mica-Fuse — Heat Coils — 
Complete Protection for Telephone Lines — The Enclosed Fuse — 
Methods of Mounting Fuses — Types of Complete Protectors. 

CHAPTER XXXI. 
Distributing Frames. ......... 612 

The Object of the Distributing Frame — Old St. Louis Frame — The Hib- 
bard Frame — The Ford and Lenfest Frame — The Cook Frame — 
The Intermediate Distributing Frame — Methods of Wiring Through 
Distributing Frame. 

CHAPTER XXXII. 

Chief Operator"s and Monitor's Equipments 628 

The Duty of Chief Operators. Monitors and Supervisors — Lines for 
Observing Service — Monitor's Taps — Lines to Local Board — Func- 
tions of Various Circuits. 






TABLE OF CONTENTS. xv 

CHAPTER XXXIII. 

PAGE 

Wire Chief's Equipment, 635 

Duty of the Wire Chief — Testing Trunks — Details of Testing Trunk Cir- 
cuits — Tests to Outside Lines — Tests to Switch-Board Lines — Mod- 
ern Testing Circuits — The Use of the Voltmeter in Testing — Direc- 
tions for Making Various Tests. 

CHAPTER XXXIV. 
The Lay-Out and Wiring of Central Office Equipments, . . 651 

Necessity for Good Workmanship — Details of Western Electric Wiring 
System — The Use of Cables in Exchange Wiring — Method of Fan- 
ning out Cables — Multiple-Jack Wiring — The Termination of Line 
Cables at the Central Office — Pot-Heads vs. Iron Box Terminals — 
Arrangement of Frames in Kellogg System — The Wiring of the 
Courtlandt Street Office — Arrangement of Multiple Cables — The 
Plaza Office in New York — Floor Plans of Several Modern Offices — 
Operators' Quarters. 

CHAPTER XXXV. 
Automatic Switch-Board Systems, 691 

Early Efforts towards Producing Automatic Switch-Boards — The Con- 
nolly-McTighe System — The Early Strowger System — The Vertical 
and Rotary Switch — The Operation of Modern Strowger System — 
The Sub-Station Equipment — Circuit of the Sub-Station Equipment 
— Vertical and Rotary Wipers — Circuits of the First Selector — The 
Side-Switch — Circuits of Second Selector — Circuits of Connector — 
Various Steps in the Connection between Two Subscribers — The Sys- 
tems at Fall River and Grand Rapids — Methods of Handling Toll 
Service — Common Battery Automatic Systems — The Work of Billi- 
ard and Rorty — Problem : The Automatic vs. The Manual Ex- 
change. 

CHAPTER XXXVI. 

Intercommunicating Systems, 736 

Interior Installations— The "House System" in General — Circuits of 
Magneto House Systems— Common Battery House Systems— The 
Ness Automatic Switch— The Holtzer-Cabot Systems— Desk Stands 
for House Systems — Metallic Circuit Systems. 



CHAPTER XXXVII. 
The Telephone Relay or Repeater, . 



"45 



The General Idea of the Repeater— Simplest Form of Repeater— Two- 
Way Repeater— The Erdman Repeater— The Stone Repeater— The 
Cooper Hewitt Mercury Vapor Repeater. 



xvi TABLE OF COXTEXTS. 

CHAPTER XXXVIII. 

FALrE 

Wire for Telephone Use. ......... 752 

Materials for Telephone Wire — Tensile Strength of Wire — Conductivity 
of Wire — The Mile-Ohm — Wire Gauges — Properties of Iron Wire — 
Galvanizing of Iron Wire — Grades of Iron Wire — Steel Wire — 
Properties of Copper Wire — Specifications for Copper Wire — In- 
sulated Wire — Magnet Wire — Rubber-Covered Telephone Wire — 
Specifications for Rubber-Covered Wire. 

CHAPTER XXXIX. 

Pole-Lixe Construction, . . .• 768 

Kinds of Wood for Telephone Poles — Life of Telephone Poles — Sizes 
of Poles — Spacing of Poles — Butt Plates — Data Concerning Loading 
of Poles on Cars — Pole Preserving Processes — Creosoting — The 
Chloride of Zinc Process — Vulcanizing — Structural Iron Poles — 
Cross Arms — Pins — Pole Hardware — Insulators — Construction 
Tools — Method of Raising and Setting Poles — Derrick Wagon — 
Method of Guying and Anchoring Messenger Wire — Clamps for 
Messenger Wire — Corner Work — Tying and Splicing of Wires — 
Sag in Wire — Telephone Circuit and Power Line. 

CHAPTER XL. 
Aerial Cable Construction, 805 

Present Tendency Toward Cable Work — Electrical and Mechanical Prob- 
lems Involved — Rubber-Covered Cable — Dry Core Paper Cable — 
Properties of Paper Cable — Two Methods of Measuring Capacity — 
"A New Danger to Lead-Covered Cables" — The Work of John 
Hesketh — The K. R. Law — Effect of K. R. Value on Transmission — 
Messenger Wire — Messenger Wire Supports — Cable Hangers — 
Methods of Stringing Cable — Cable Splicing — Pot-Head Terminals 
— Iron-Box Heads — Multiple Cable Taps — Method of Making 
Y- Splice — Method of Distributing from Terminal Poles. 

CHAPTER XLI. 
Underground Cable Construction, ....... 835 

Advantage of Placing Telephone Wire Underground — Requirements of 
Conduit — Different Kinds of Conduit — Methods of Laying Conduit — 
Bends to Avoid Obstacles — Rodding — The Drawing in of Cable — 
Steam Winch — Electric Winch — Cable Reel Truck — Data Concern- 
ing Cost of Conduit. 

CHAPTER XLII. 

Testing, 851 

Classification of Tests — Tests with Magneto Set — Tests with Telephone 
Receiver — The Wheatstone Bridge — Commercial Forms of Bridges — 
The Galvanometer — Types of Galvanometers — Methods of Losing 
Shunt — Obtaining Constant of Galvanometer — Measurement of In- 
sulation Resistance — Capacity Test? — Location of Faults — The Var- 
lev Lnoo Test — The Murray Lood Test — Voltmeter Tests. 



AMERICAN TELEPHONE PRACTICE. 



CHAPTER I. 

HISTORY AND PRINCIPLES OF THE MAGNETO TELEPHONE. 

The history of the telephone, from its inception to its present 
state of perfection, is interesting in the extreme, and affords a 
striking example of the fact that great inventions are almost in- 
variably the result of long and careful study on the part of many 
workers, rather than the sudden inspiration of a single genius. 
It is of even greater interest from a scientific standpoint, for in 
no way can one obtain a better idea of the fundamental princi- 
ples involved in telephony than by following their development, 
step by step, noting the contributions made by each of the many 
scientists and inventors whose names are closely connected with 
electrical progress. 

These steps were made in logical order, the knowledge con- 
tributed by each investigator making possible a deeper insight into 
the subject on the part of his successors. It is best, therefore, to 
follow this order in obtaining primary ideas of the subject. 

The history of the knowledge of electromagnetism begins with 
July 20, 1820, and with this date very properly begins the history 
of the electric telephone. On that day Oersted, a professor in the 
University of Copenhagen, discovered that a magnetic needle tends 
to place itself at right angles to a wire carrying a current of elec- 
tricity. Ampere immediately took up the subject, and in a very 
short time disclosed the laws upon which present electromagnetic 
theory is based. 

In the following year Arago and Davy discovered that if a cur- 
rent be caused to flow through an insulated wire wrapped about 
a rod of steel the latter would exhibit magnetic properties. It was 
William Sturgeon, however, who in 1825 made an electromagnet- 
as we know it to-day, and called it by that name. To these three 
men, therefore, belongs the credit of one of the greatest discoveries 
in the history of science. Joseph Henry also made his classic ex- 
periments on the electromagnet, and to him must be accredited a 



2 AMERICAN TELEPHONE PRACTICE. 

large amount of our knowledge regarding it. Henry showed how 
to build a magnet capable of being operated over a great length of 
wire, a most important step. 

In 1 83 1 Faraday and Henry, independently, discovered the con- 
verse of these laws of electromagnetism — that if the intensity of a 
magnetic field inclosed by a conductor be in anywise changed, a 
current of electricity will flow in the conductor. This current will 
flow only while such change is taking place, and its strength will 
depend directly on the rate of the change. 

These two laws concerning the transformation of electric energy 
into magnetic, and its converse, the transformation of magnetic 
energy into electric, are certainly the most important in the whole 
realm of electrical science; as singly or together they form the 
foundations not only of the telephone and telegraph, but of electric 




II 



FIG. l.-STURGEON ELECTROMAGNET. 



lighting, electric power transmission, and of every other achieve- 
ment by which electricity has revolutionized the methods of life 
throughout the whole civilized world. 

As these laws form the very root of all telephone practice, a few 
illustrations directly in line with the principles of the telephone will 
not be amiss, even though they are very generally understood; for 
they will give a clearer understanding of the developments made by 
subsequent inventors. 

If, as shown in Fig. 1, a coil of wire be wrapped around a rod, R, 
of iron or steel, and a battery, B, placed in circuit with the coil, the 
rod becomes a magnet upon the closure of this circuit, and will 
attract an iron armature, A, in the vicinity of either of its poles. 
Any variation in the strength of this current will cause corre- 
sponding variations in the attractive power of the magnet. If the 
rod be of steel, and permanently magnetized, it will exert an attract- 



THE MAGNETO TELEPHONE. 



ive force of its own on the armature, and the current will, according 
to its direction, increase or diminish this attractive force 

About every magnet there exists a field of force; that is. a 
region in which any body capable of being magnetized (such as 




FIG. 2.— LINES OF FORCE OF BAR-MAGNET. 

iron) has exerted on it, by the magnet, an influence of attraction 
or repulsion. This field of force is usually graphically repre- 
sented by closed curves, radiating from the poles of the magnet, 
and the strength of the magnet is commonly measured in terms of 




Q> 



FIG. 3.-FARADAY AND HENRY MAGNETO ELECTRICITY 



the number of such lines radiating from one of its poles. A magnet 
may be made to map out its own field of force by placing it in a 
horizontal position and directly over it a sheet of paper or card- 
board. If iron filings are then dropped from a height of a few foot. 



4 AMERICAN TELEPHONE PRACTICE. 

on the paper, they will arrange themselves in the direction of the 
lines of force. Fig. 2 shows such a map produced by the bar- 
magnet, N S. 

If now a galvanometer, G, or other current-indicator (Fig. 3) be 
placed in circuit with a coil, C, and a magnet N S, moved in the 
vicinity of the coil, or the coil in the vicinity of the magnet, in such 
manner as to change the number of the lines of force passing 
through the coil, a current is generated in the coil and is indicated 
by the galvanometer. This current will flow only while such move- 
ment is taking place. Its direction will depend on the direction of 
the lines of force threading the coil and on whether their number 
is being increased or diminished. Its strength will depend on the 
rate at which their number is changing. 

If a mass of iron be brought within the field of a magnet, the field 
becomes distorted by virtue of a larger number of lines finding their 
path through the space occupied by the iron than through the same 
space when filled with air. Therefore, if a closed coil be placed 




FIG. 4.— MORSE ELECTROMAGNETIC TELEGRAPH. 



about a pole of the magnet and the body of iron be moved to and 
from the pole, the intensity of the field in which the coil lies will 
vary, and currents of electricity will flow in the coil. 

In 1837 Professor Page of Salem, Mass., discovered that a rod 
of iron, suddenly magnetized or demagnetized, would emit certain 
sounds due to a molecular rearrangement caused by the changing 
magnetic conditions. This phenomenon is known as "Page's 
effect." 

Late in the thirties Professor S. F. B. Morse placed at one end 
of a line Sturgeon's electromagnet, M (Fig. 4), with a pivoted arma- 
ture, A, and at the other end a battery, B, and a key, K, for making 



THE MAGNETO TELEPHONE. 5 

and breaking the circuit. By manually closing and opening the 
key, the core of the magnet became magnetized and demagnetized, 
thus alternately attracting and releasing the armature. By this 
means signals were sent and recorded on a strip of paper, carried 
on a roller, R, in front of the armature, and thus intelligence was 
practically conveyed by electrical means between distant points. 

In 1854 a Frenchman, Charles Bourseul, predicted* the trans- 
mission of speech, and outlined a method correct save in one par- 
ticular, but for which error one following his directions could have 
produced a speaking telephone. His words at this date seem al- 
most prophetic: 

"I have asked myself, for example, if the spoken word itself could 
not be transmitted by electricity; in a word, if what was spoken in 
Vienna may not be heard in Paris ? The thing is practicable in this 
way: 

"Suppose that a man speaks near a movable disc, sufficiently flex- 



X 



& 



FIG. 5.— REIS' MAKE-AND-BREAK TELEPHONE. 

ible to lose none of the vibrations of the voice ; that this disc alter- 
nately makes and breaks the connection from a battery : you may 
have at a distance another disc which will simultaneously execute the 
same vibrations." 

The words "makes and breaks" in Bourseul's quotation have been 
italicized by the present writer. They form the keynote of the fail- 
ures of those who subsequently followed Bourseul's directions 
literally. 

Philip Reis, a German inventor, constructed what he called a 
telephone in 1861, following implicitly the path outlined by Bourseul. 
He mounted a flexible diaphragm, D (Fig. 5), over an opening in a 
wooden box, and on the center of the diaphragm fastened a small 



*Vol. XXIV., "L'lllustration," Paris, Aug.. 20. ISM. 



6 AMERICAN TELEPHONE PRACTICE. 

piece of platinum. P. Xear this he mounted a heavy brass spring, 
s, with which the platinum alternately made and broke contact when 
the diaphragm was caused to vibrate. These contact points formed 
the terminals of a circuit containing a battery, B, and the receiving 
instrument. His receiver assumed various forms, prominent among 
which was a knitting, needle, N, wrapped with silk-insulated copper 




FIGS. 6 AND 7.— REIS' TELEPHONE TRANSMITTER AXD RECEIVER. 

wire and mounted on a cigar box for a sounding board. Its opera- 
tion was as follows : The sound waves set up in the air struck against 
the diaphragm of the transmitter, causing it to vibrate in unison with 
them. This caused the alternate making and breaking of the circuit 
at the point of contact between the platinum and the spring, and 
allowed intermittent currents to flow through the receiver. These 




FIG. 8.— SOUND WAVES OF VOICE AXD SIMPLE MUSICAL NOTE. 



caused a series of sounds in the knitting needle by virtue of ''Page's 
effect." The sounding board vibrated in unison with the molecular 
vibrations of the needle, and the sound was thus greatly amplified. 
Reis' transmitter and one form of his receiver are shown in Figs. 
6 and 7, respectively. 

Reis' telephone could be depended upon to transmit only musical 
sounds. The question as to whether it actually did transmit speech 



THE MAGNETO TELEPHONE. 7 

has been the subject of much discussion, but if it did this at all it was 
very imperfectly. The cause of its failure to successfully transmit 
speech will be understood from the following facts : 

A simple musical tone is caused by vibrations of very simple forms, 
while sound waves produced by the voice in speaking, are very com- 
plex in their nature. These two forms of waves may be graphically 
represented as in Fig. 8. 

Sound possesses three qualities : pitch, depending entirely on 
the frequency of the vibrations ; loudness, depending on the ampli- 
tude of the vibrations, and timbre or quality, depending on the form 
of the vibration. The tones of a flute and a violin may be the same 
as to pitch and loudness and yet be radically different. This differ- 
ence is in timbre or quality. 

Reis' transmitter, as he adjusted it, was able only to make and 
break the circuit, and a movement of the diaphragm barely sufficient 




BELL MAGNETO TELEPHONE. 



to break the circuit produced the same "effect as a much greater 
movement. The current therefore flowed with full strength until the 
circuit was broken, when it stopped entirely. The intermediate 
strengths needed for reproducing the delicate modulations of the 
voice were entirely lacking. This apparatus could therefore exactly 
reproduce the pitch of a sound, but not its timbre and relative loud- 
ness. 

For the next fifteen years no apparent advance was made in the 
art of telephony, although several inventors gave it their attention. 

In 1876 Professor Alexander Graham Bell and Professor Elisha 
Gray almost simultaneously invented successful speaking telephones. 
Gray has been one of the principal claimants for the honor of being 
the first inventor of the telephone, but Bell has apparently estab- 
lished his right to it, and has also reaped the profit, for, after long 
litigation, the United States Patent Office and the courts have 
awarded the priority to him as against Gray and many others. 



AMERICAN TELEPHONE PRACTICE. 



Bell possessed a greater knowledge of acoustics than of electrical 
science, and it was probably this that led him to appreciate wherein 
others had failed. His instrument consisted of a permanent bar- 
magnet, B (Fig. 9), having on one end a coil of fine wire. In front 
of the pole carrying the coil a thin diaphragm, D, of soft iron was so 




FIG. 11.— BELL'S CENTENNIAL TRANSMITTER. 



Two of 



mounted as to allow its free vibration close to the pole, 
the instruments are shown connected in a circuit in Fig, 9. 

Two points will be noticed which have heretofore been absent: 
that no battery is used in the circuit, and that the transmitting and 



THE MAGNETO TELEPHONE. 9 

receiving instruments are exactly alike. When the soft-iron dia- 
phragm of the transmitting instrument is spoken to, it vibrates in 
exact accordance with the sound waves striking against it. The 
movement of the diaphragm causes changes in the magnetic field 
in which lies the coil, which changes, as already pointed out, cause 
currents to flow in the circuit. These currents flow first in one 
direction, and then in the other, varying in unison with the move- 
ments of the diaphragm, the waves being very complex, and repre- 
sented graphically, similar to those of the voice shown in Fig. 8. 
Passing along the line wire, these electrical impulses, so feeble that 
only the most delicate instruments can detect them, alternately in- 
crease and decrease the strength of the permanent magnet of the re- 
ceiving instrument, and thereby cause it to exert a varying pull on 
its soft-iron diaphragm, which, as a result, takes up the vibrations 
and reproduces the sound faithfully. Bell's earlier instruments, 
exhibited in 1876 at the Centennial in Philadelphia, are shown in 
Figs. 10 and 11, the former being his receiver, the latter his trans- 
mitter. The receiver consisted of a tubular magnet, composed of a 
coil of wire, H, surrounding a core, C, and inclosed in an iron tube, 
E, which was about if inches in diameter and 3 inches long. This 
tube was closed by a thin iron armature or diaphragm, D, which 
rested loosely on the upper face of the iron tube, the length of the 
core being such as not quite to touch the diaphragm when in this 
position. The whole was mounted on a base, F, as shown, arrange- 
ments being made to adjust the air gap between the pole of the core 
and the diaphragm by means of a thumb-screw. 

The transmitter, Fig. 11, consisted of an electromagnet, H, in 
front of the core, C, of which was adjustably mounted a diaphragm 
of goldbeater's skin, D y carrying a small iron armature, A, at its 
center. A long mouthpiece, M, into which the sounds to be trans- 
mitted were spoken, served to convey the sound waves more directly 
to the diaphragm. 

Nearly all books and articles on telephones, that treat of Bell's 
early receiver at all, show and describe it as having the diaphragm 
fastened at one edge by a single small screw to the upper face of the 
iron tube, and sprung away from the tube at its opposite side. This 
mistake occurred in the first two editions of this work, and would 
have been in this one but for Mr. Thomas D. Lockwood. who was 
kind enough to call attention to it. The origin of the error is ex- 
plained in the following interesting extract from a letter written by 
Mr. Lockwood to the writer of this book : 



10 



AMERICAN TELEPHONE PRACTICE. 



"This mistake first appeared in the account given by Engineering 
of Sir William Thomson's address to the British Association in 
September, 1876, and has been universally copied. * * * The 
origin of the mistake is very odd. The screw of the instrument 
given to Sir William Thomson, and which he exhibited in England 
on his return, was put through a hole in the edge of the diaphragm, 
and engaged with a threaded hole in the edge of the tube, for the 
purpose of attaching the diaphragm while in transit, to prevent it 
from getting lost. No one, however, notified Sir William of this, it 
probably having been forgotten ; and Sir William seems to have for- 
gotten what the instrument, as he saw it in Philadelphia, looked 
like. Finally, in kocking about among Sir William's luggage, the 
free end of the diaphragm was apparently, and without doubt unin- 




FIG. 12.-ROYAL E. HOUSE'S ELECTRO-PHONETIC TELEGRAPH. 

tentionally, bent upward, as the picture shows. But when so bent, 
being at the same time rigidly fastened at the opposite edge, it would 
not and could not work : and when Sir William showed it in England 
he couldn't make it work." 

Bell's instrument, in a modified form, is the standard of to-day. 
It is now used as a receiver only, a more efficient transmitter, de- 
pending upon entirely different principles, having been invented. 

In speaking of Bell's invention, Sir William Thomson, now Lord 
Kelvin, has said: "Who can but admire the hardihood of invention 
which devised such very slight means to realize the mathematical 
conception that if electricity is to convey all the delicacies of quality 



THE MAGNETO TELEPHONE. 11 

which distinguish articulate speech, the strength of its current must 
vary continuously as nearly as may be in simple proportion to the 
velocity of a particle of air engaged in constituting the sound ?" 

Much has been said, and books have been written on the rights 
of Reis as the inventor of the speaking telephone. The validity of 
Bell's controlling patent 1 was the subject of. many attacks, the litiga- 
tion finally reaching the Supreme Court of the United States. In 
the opinion of this court (October Term 1887) 2 the following brief 
but comprehensive statement is found: 

"We have not had our attention called to a single item of evidence 
which tends in any way to show that Reis or any one who wrote 
about him had it in his mind that anything else than the intermit- 
tent current caused by the opening and closing of the circuit could 
be used to do what was wanted. No one seems to have thought 
that there could be another way. All recognized the fact that the 
'minor differences in the original vibrations' had not been satisfac- 
torily reproduced, but they attributed it to the imperfect mechanism 
of the apparatus used, rather than to any fault in the principle on 
which the operation was to depend. 

"It was left for Bell to discover that the failure was due not to 
workmanship, but to the principle which was adopted as the basis 
of what had to be done. He found that what he called the intermit- 
tent current — one caused by alternately opening and closing the 
circuit — could not be made under any circumstances to repro- 
duce the delicate forms of the air vibrations caused by the human 
voice in articulate speech, but that the true way was to operate on 
an unbroken current by increasing and diminishing its intensity. 
* * * Such was his discovery, and it was new. Reis never 
thought of it, and he failed to transmit speech telegraphically. Bell 
did and he succeeded. Under such circumstances it is impossible 
to hold that what Reis did was an anticipation of the discovery of 
Bell. To follow Reis is to fail, but to follow Bell is to succeed. 
The difference between the two is just the difference between failure 
and success." 

A very interesting fact, and one which might have changed the 
entire commercial status of the telephone industry is that in 1868 
Royal E. House, of Binghamton, N. Y., invented and patented an 
"electro-phonetic telegraph," which was capable of operating as a 
magneto telephone, in the same manner as the instruments subse- 



1 United States Patent No. 174,405, dated March 7, 1876; application filed February 
14, 1876. 

2 Supreme Court Decisions, United States reports. Volume 12t>. 



12 AMERICAN TELEPHONE PRACTICE. 

quently devised by Bell. House knew nothing of its capabilities, 
however, unfortunately for him. The instrument is shown in Fig. 
12, and is provided with a sounding diaphragm of pine wood stiff- 
ened with varnish, mounted in one end of a large sound amplifying 
chamber, so formed as to focus the sound waves at a point near its 
mouth, where the ear was to be placed to receive them. The electro- 
magnet adapted to be connected in the line circuit had its armalure 
connected by a rod with the center of the wooden diaphragm as 
shown. By this means any movements imparted to the armature 
by fluctuating currents in the line were transmitted to the dia- 
phragm, causing it to give out corresponding sounds; and any 
movements imparted to the diaphragm by sound waves were trans- 
mitted to the armature, causing its movements to induce correspond- 
ing currents in the line. Two of these instruments connected in a 
circuit, as shown in Fig. 9, would act alternately as transmitters and 
receivers in the same manner as Bell's instruments. 



CHAPTER II. 

HISTORY AND PRINCIPLES OF THE VARIABLE RESISTANCE 

TRANSMITTER. ' 

It has been shown that in order to transmit speech by electricity 
it is necessary to cause an undulatory or alternating current to flow 
in the circuit over which the transmission is to be effected, and that 
the strength of this current must at all times be in exact accordance 
with the vibratory movements of the body producing the sound. 

Bell's magneto transmitter was used as the generator of this 
current ; as a dynamo, in fact, the energy for driving which was de- 
rived from the sound waves set up by the voice. The amount of 
energy so derived was, however, necessarily very small and the cur- 
rent correspondingly weak, and for this reason this was not a prac- 
tical form of transmitter, except for comparatively short lines. 

Elisha Gray devised a transmitter which, instead of generating 
the undulatory current itself, depended for its action on causing 
variation in the strength of a current generated by some separate 
source ; this variation in current strength always being in accord- 
ance with the movements of the diaphragm. 

He mounted on his horizontal vibrating diaphragm a metal needle, 
extending into a fluid of low conductivity, such as water. The needle 
formed one terminal of the circuit, the other terminal being a metal 
pin, extending up through the bottom of the containing vessel. The 
vibration of the diaphragm was supposed to cause changes in the 
resistance of the path through the fluid on account of the varying 
distance between the points of the electrodes and therefore corre- 
sponding changes in the strength of the current. 

Bell also used a liquid transmitter (Fig. 13) in which a conduct- 
ing liquid was held in a conducting vessel, C, forming one terminal 
of the circuit. The other terminal was a short metallic needle, R, 
carried on the diaphragm, D, and projecting slightly into the liquid. 
so that the area of contact between the liquid and the needle would be 
varied to better advantage by the vibration of the diaphragm than if 
the needle were immersed a greater distance into the fluid. 

Bell's liquid transmitter depended on variation in the extent of 
immersion of the electrode, while Gray's instrument, owing to the 

13 



14 



AMERICAN TELEPHONE PRACTICE. 



great extent to which the pin was immersed, depended rather on 
the variation in the length of the conducting path through the liquid 
itself, a faulty principle for this purpose. 

Bell's liquid transmitter was also exhibited at the Philadelphia 




FIG. 13.— BELL'S CENTENNIAL LIQUID TRANSMITTER. 

Centennial in 1876, and, unlike that of Reis, simply caused variations 
in the resistance of the circuit, and thereby allowed a continuous 
but undulatory current to pass over the line, the variations in which 
were able to reproduce all the delicate shades of timbre, loudness, 
and pitch necessary in articulate speech. 

Gray and Bell embodied, or attempted to embody, in these instru- 
ments the main principle upon which all successful battery transmit- 
ters are based. A battery furnished the current, and the transmitter, 
actuated by the voice, served to modulate it. It was not long, how- 
ever, before a much better means was devised for putting this 
principle into practice. 

In 1877 Emile Berliner, of Washington, D. C, filed a caveat, and 
later in the same year applied for a patent on a transmitter, depend- 
ing upon a principle pointed out in articles published in 1856, 1864 
and 1874 by the French scientist, Du Moncel, that if the pressure 



THE BATTERY TRANSMITTER. 



15 



between two conducting bodies forming part of an electric circuit 
be increased, the resistance of the path between them will be dimin- 
ished, and conversely, if the pressure between them be decreased, 
a corresponding increase of resistance will result. 

Berliner's transmitter is shown in principle in Fig. 14, which is a 
reproduction of the principal figure in his now famous patent. In 
this A is the vibratory diaphragm of metal, against the center of 
which rests the metal ball, C, carried on a thumb-screw, B, which is 
mounted in the standard, d. The pressure of the ball, C, against 
the plate, A, can be regulated by ^turning the thumb-screw. The 
diaphragm and ball form the terminals or electrodes of a circuit, 




FIG. 14.— BERLINER'S TRANSMITTER. 



including a battery and receiving instrument. Figs. 15 and 16 
show two different views of an exact duplicate of Berliner's original 
model as filed in the patent office. This was very roughly con- 
structed as shown. The diaphragm was a circular piece of ordi- 
nary tin and the contact-piece a common blued-iron wood screw. 
The action of this instrument (which at best has never been satis- 
factory or commercial) is as follows: when the diaphragm vibrates, 
the pressure at the point of contact, a, Fig. 14, becomes greater or 
less, thus varying the resistance of the contact and causing corre- 
sponding undulations in the current flowing. 

Soon after this Edison devised an instrument using carbon as the 
medium for varying the resistance of the circuit with changes of 
pressure. Edison's first type of carbon transmitter consisted simply 
of a button of compressed plumbago bearing against a small plati- 



16 



AMERICAN TELEPHONE PRACTICE. 





FIGS. 15 AND 16.— BERLINER'S PATENT-OFFICE MODEL. 



THE BATTERY TRANSMITTER. 



17 



num disc secured to the diaphragm. The plumbago button was 
held against the diaphragm by a spring, the tension of which could 
be adjusted by a thumb-screw. 

A form of Edison's transmitter, devised by George M. Phelps in 
1878, is shown in Fig. 17. The transmitting device proper is shown 




FIG. 17.— PHELPS-EDISON TRANSMITTER. 



in the small cut at the right of this figure, and is inclosed in a cup- 
shaped case formed of the two pieces, A and B, as shown. Secured 
to the front of the enlarged head, e, of the adjustment screw, E, is a 
thin platinum disc, i 7 , against which rests a cylindrical button, G, 
of compressed lampblack. A plate of glass, /, carrying a hemi- 
spherical button, K, has attached to its rear face another platinum 
disc, H. This second platinum disc rests against the front face of 
the lampblack disc, G, and. the button, K, presses firmly against the 
center of the diaphragm, D. The plates, F and H, form the ter- 
minals of the transmitter, and as the diaphragm, D, vibrates, it 
causes variations in the pressure, and corresponding changes in the 
resistance of the circuit, thus producing the desired undulations of 
current. 

Professor David B. Hughes made a most valuable contribution 
tending toward the perfection of the battery transmitter. By a 
series of interesting experiments, he demonstrated conclusively that 
a loose contact between the electrodes, no matter of what substance 
they are composed, is far preferable to a firm, strong contact. The 
apparatus used in one of his earlier experiments, made in 1878, is 
shown in Fig. 18, and consists simply of three wire nails, of which 
A and B form the terminals of the circuit containing a battery and 
a receiving instrument. The circuit was completed by a third nail, 
C, which was laid loosely across the other two. Any vibrations in 
the air in the vicinity caused variations in the intimacy of contact 



18 



AMERICAN TELEPHONE PRACTICE. 



between the nails, and corresponding variations in the resistance 
of the circuit. This was a very inefficient form of transmitter, but 
it demonstrated the principle of loose contact very cleverly. 

It was found that carbon was, for various reasons, by far the 
most desirable substance for electrodes in the loose contact trans- 
mitter, and nothing has ever been found to even approach it in 
efficiency and desirability. 

Another form of transmitter devised by Hughes, and called by 
him the microphone, is shown in Fig. 19. This consists of a small 
pencil of gas carbon, A, pointed at each end, and two blocks, B B, 
of carbon fastened to a diaphragm or sounding board, C. These 
blocks are hollowed out in such a manner as to loosely hold between 
them the pencil, A. The blocks, B B, form the terminals of the cir- 
cuit. This instrument, though crude in form, is of marvelous deli- 




FIG. 18.— HUGHES' NAIL MICROPHONE. 



cacy and is well termed microphone. The slightest noises in its 
vicinity, and even those incapable of being heard by the ear alone, 
produce surprising effects in the receiving instrument. This partic- 
ular form of instrument is, in fact, too delicate for ordinary use, as 
any jar or loud noise will cause the electrodes to break contact and 
produce deafening noises in the receiver. Nearly all carbon trans- 
mitters of to-day are of the loose-contact type, this having entirely 
superseded the first form devised by Edison, which was then sup- 
posed to depend on the actual resistance of a carbon block being 
changed under varying pressure. 

In speaking of Professor Hughes' work on loose contacts and the 



THE BATTERY TRANSMITTER. 



19 



microphone, the Telegraph Journal and Electrical Review, an Eng- 
lish electrical paper, says in its issue of July I, 1878: "The micro- 
phone is a striking illustration of the truth that in science any phe- 
nomenon whatever may be turned to account. The trouble of one 
generation of scientists may be turned to the honor and service of 
the next. Electricians have long had sore reasons for regarding a 
'bad contact' as an unmitigated nuisance, the instrument of the evil 
one, with no conceivable good in it, and no conceivable purpose ex- 
cept to annoy and tempt them into wickedness and an expression of 
hearty but ignominious emotion. Professor Hughes, however, has, 
with a wizard's power, transformed this electrician's bane into a 
professional glory and a public boon. Verily, there is a soul of vir- 
tue in things evil." 

Professor Hughes, in an article in Nature, June 27, 1878, thus 




FIG. 19.— HUGHES' CARBON MICROPHONE. 



describes the conditions necessary for microphonic action : "If the 
pressure on the materials is not sufficient, we shall have a constant 
succession of interruptions of contact, and the galvanometer needle 
will indicate the fact. If the pressure on the materials is gradually 
increased the tones will be loud but wanting in distinctness, the 
galvanometer indicating interruptions ; as the pressure is still in- 
creased, the tone becomes clearer, and the galvanometer will be sta- 
tionary when a maximum 01 loudness and clearness is attained. If 
the pressure be further increased, the sounds become weaker, though 
very clear, and, as the pressure is still further augmented, the sounds 
die out (as if the speaker was talking and walking away at the same 
time) until a point is arrived at where there is complete silence." 



20 



AMERICAN TELEPHONE PRACTICE. 



Only one radical improvement now remains to be recorded. In 
1881 Henry Hunnings devised a transmitter wherein the variable 
resistance medium consisted of a mass of finely divided carbon 
granules held between two conducting plates. His transmitter 
is shown in Fig. 20. Between the metal diaphragm, A, and a 
parallel conducting plate, B, both of which are securely mounted 
in a case formed by the block, D, and a mouthpiece, F, is a chamber 
filled with fine granules of carbon, C. The diaphragm, A, and the 
plate, B, form the terminals of the transmitter, and the current from 
the battery must therefore flow through the mass of granular car- 
bon, C. When the diaphragm is caused to vibrate by sound waves, 
it is brought into more or less intimate contact with the carbon 
granules and causes a varying pressure between them. The resist- 




FIG. 20.— HUNNING'S GRANULAR CARBON TRANSMITTER. 



ance offered by them to the current is thus varied, and the desired 
undulations in the current produced. This transmitter, instead of 
having one or a few points of variable contact, is seen to have a 
multitude of them. It can carry a larger current without heating, 
and at the same time produce greater changes in its resistance, than 
the forms previously devised, and no ordinary sound can cause a 
total break between the electrodes. These and other advantages 
have caused this type in one form or another to largely displace all 
others. 

At first the practice was to put the transmitter, together with the 
receiver and battery, directly in circuit with the line wire. With 



THE BATTERY TRANSMITTER. 21 

this arrangement the changes produced in the resistance by the 
transmitter were small in comparison with the total resistance of 
the circuit, especially in the case of a long line, and the changes in 
current were therefore small. Edison remedied this difficulty by 
using an induction coil in connection with the transmitter. 

The induction coil used then and now is made as follows : Around 
a core formed of a bundle of soft iron wires is wound a few turns of 
comparatively heavy insulated copper wire. Outside of this, and 
entirely separate from it, is wound another coil consisting of a 
great number of turns of fine wire, also of copper, and insulated. 
The transmitter, together with the battery, is placed in a closed cir- 
cuit with the coarse winding of a few turns, while the fine winding 
of many turns is included directly in circuit with the line wire and 
the receiving instrument. The coarse winding is usually termed the 
primary winding, because it is associated with the primary source 
of current, the battery ; while the fine winding is usually termed the 
secondary winding, because the currents flowing in it at the trans- 
mitting station are secondary, or induced currents. In coils of this 
kind the coarse winding is* almost invariably termed the primary 
for the above reason, although many conditions exist in electrical 
work, and in telephone work, where the high resistance winding is in 
reality the primary coil. 

The circuit arrangement spoken of is shown in Fig. 21, in which 
T is a transmitter, B a battery, P and 5 primary and secondary 




FIG. 21.— TRANSMITTER WITH INDUCTION COIL. 

windings, respectively, of an induction coil, L L' the line wires, and 
R the receiving instrument. It is well to state here that the usual 
way of indicating the primary and secondary of an induction coil, in 
diagraphic representation of electrical circuits, is by an arrange- 
ment of two adjacent zigzag lines, as shown in Fig, 21, A current 



22 AMERICAN TELEPHONE PRACTICE. 

flowing in the primary winding of the induction coil produces a 
field of force in the surrounding space, and any changes caused 
by the transmitter in the strength of the current produce changes in 
the intensity of this field. As the secondary winding lies in this field, 
these changes will, by the laws of Faraday and Henry, cause cur- 
rents to flow in the secondary winding and through the line wire 
to the receiving instrument. In good induction coils the electro- 
motive forces set up in the secondary coil bear nearly the same ratio 
to the changes in electromotive force in the primary coil, as the 
number of turns in the secondary bears to the number of turns in 
the primary. 

The use of the induction coil with the transmitter accomplishes 
two very important results : First, it enables the transmitter to ope- 
rate in a circuit of very low resistance, so that the changes in the 
resistance produced by the transmitter bear a very large ratio to 
the total resistance of the circuit. This advantage is well illustrated 
by contrasting the two following cases : 

Suppose a transmitter capable of producing a change of resistance 
of one ohm be placed directly in a line circuit whose total resistance 
is iooo ohms; a change in the resistance of the transmitter of one 
ohm will then change the total resistance of the circuit one one-thou- 
sandth of its value, and the resulting change in the current flowing 
will be but one one-thousandth of its value. On the other hand, sup- 
pose the same transmitter to be placed in a local circuit, as above de- 
scribed, the total resistance of which circuit is 5 ohms ; the change of 
one ohm in the transmitter will now produce a change of resistance 
of one-fifth of the total resistance of the circuit, and cause a change 
of one-fifth of the total current flowing. It is thus seen that fluctua- 
tions in the current can be produced by a transmitter with the aid 
of an induction coil which are many times greater than those pro- 
duced by the same transmitter without the coil. 

The second advantage is that by virtue of the small number of 
turns in the primary winding and the large number in the secondary 
winding of the induction coil, the currents generated in the sec- 
ondary are of a very high voltage as compared with those in the 
primary, thus enabling transmission to be effected over much greater 
length of line, and over vastly higher resistances than would be pos- 
sible if the transmitter were forced to vary the current flowing 
through the entire length of the line. 



CHAPTER III. 

ELECTROMAGNETIC AND ELECTROSTATIC INDUCTION. 

Induction — both electromagnetic and electrostatic — together 
with the allied subjects — self-induction, retardation, and capacity — 
play such important parts in the whole telephonic art, and seem so 
little understood among the rank and file of telephone workers and 
users, that this chapter will be devoted to an elementary and, as far 
as possible, non-mathematical discussion of these two phenomena, 
with a view to explaining their existence and effect in a simple 
manner, rather than to throw any new light upon the subject. 

Ohm's law states that for a steady flow of electricity in a given 
circuit the amount of current in amperes is equal to the electro- 
motive force expressed in volts, divided by the resistance of the 
circuit expressed in ohms. In algebraic form this becomes the 
well-known equation: 

where I represents the current in amperes, E the electromotive 
force in volts, and R the resistance of the circuit in ohms. Knowing 
any two of the above quantities, the third may be determined from 
the equation already given, or from the following, which are derived 
from it: 

E — IR t 

and R— -=-. 

These three equations, which are merely different ways of ex- 
pressing Ohm's law, are the most useful in the entire science of 
electricity. It is unfortunate for an easy understanding of tele- 
phony that these equations in their simple forms hold true for a 
steady How only, and that when currents which are rapidly changing 
in value or in direction are considered, we must face a more com- 
plex set of conditions. 

An electric current flowing through a conductor sets up a field of 
force about the conductor throughout its entire length. This held 
of force consists of magnetic lines extending in closed curves about 
the conductor, and is often termed a magnetic whirl A freely 
suspended magnetic needle placed within this field of force will tend 



24 AMERICAN TELEPHONE PRACTICE. 

to assume a direction corresponding to the direction of the lines of 
torce, and therefore at right angles to the conductor. 

If the current flowing in the conductor is maintained at a constant 
value and in the same direction, the field of force about the conduc- 
tor will not change. On the other hand, if the current strength 
fluctuates, the field of force will become more intense and will ex- 
pand while the current strength is increasing, and will become less 
intense and will therefore contract while the current strength is 
decreasing. If the current changes its direction, the field of force 
existing is entirely destroyed, and is built up in an opposite 
direction. 

Whenever there is such a relative movement between a conductor 
and the lines of force of a magnetic field as to cause the conductor to 
cut the lines, or the lines to cut the conductor, an electromotive 
force is set up in the conductor which tends to cause a current to 
flow. This direction of the electromotive force will depend on the 
direction of the lines and of the movement of the conductor, and 
its value will depend on the rate of cutting. The field of force may 
be set up either by a magnet or by a conductor carrying a current, 
and in either case the phenomenon just described is called electro- 
magnetic induction. 

If two wires, one of them carrying a current, are formed into 
adjacent parallel coils, each having a number of turns, then the 
lines of the field of force set up by the coil carrying the current will, 
when the field of force contracts or expands, cut some or all of the 
turns of the other coil. An electromotive force will thus be induced 
in each turn of the latter coil, the result being that the sum of all 
the electromotive forces induced in the separate turns will be added, 
thus producing a much greater effect than if the latter coil had but 
a single turn. The contracting or expanding of the field due to 
the coil carrying the current will take place only when the current 
in this coil is decreasing or increasing in value, and it therefore 
follows that electromotive forces will be induced in the second coil, 
only when the current in the first coil is changing. Furthermore, 
if the two coils are wrapped about an iron core, the field of force due 
to the coil carrying the current will be greatly strengthened, because 
a given magnetizing force, or force which tends to set up a field of 
force, will produce a greater number of lines in iron than in air. 
The electromotive force induced in the second coil will therefore 
be greatly increased, owing to the greater rate of cutting of lines 
caused by the changes in the current. 



INDUCTION AND CAPACITY. 25 

It is evident that in a coil formed of a single wire, carrying a cur- 
rent, each turn of the coil is surrounded by a field of force, and that 
each turn must therefore lie more or less within the fields of force 
of all the other turns. Each turn will therefore have an inductive 
action upon all the other turns of the same coil when the current 
through the coil is varying. Whenever a diminution of the current 
occurs the decreasing number of lines of force set up by any one 
turn will act on each of the other turns to induce an electromotive 
force tending to cause a current to flow in the same direction. The 
decreasing field of force around each one of the turns will act in a 
like manner on all of the other turns, and as all of the electromotive 
forces in all of the turns will be in the same direction as the current 
which is already flowing in the coil, their effects will be added and 
Avill tend to prolong the flow of current. On the other hand, an 
increase in the current will cause an increasing number of lines to 
surround each turn, and this increase around any one turn will 
induce electromotive forces in each of the other turns in the opposite 
direction to that producing the current already flowing. This phe- 
nomenon of induction between the various parts of a single coil of 
wire each on the other is termed self-induction. 

In view of the fact that a decreasing current induces an electro- 
motive force tending to produce a current in the same direction as 
that already flowing, while an increasing current induces an electro- 
motive force tending to produce current in the opposite direction, 
it follows that the general effect of self-induction in a circuit is to 
tend to prevent any changes in current from taking place in that 
circuit. This accounts for the fact that coils of wire, such as those 
forming electromagnets, tend to so greatly retard the flow of any 
rapidly varying currents, such as voice currents, through them. 

It is quite evident that in circuits containing self-induction and 
subject to rapidly fluctuating electromotive forces, the tendency of 
self-induction to prevent changes in the current will always cause 
any change in current to lag slightly behind the change in electro- 
motive force which produces it. Where the electromotive force 
impressed upon a circuit varies according to the law of sines, the 
electromotive force produced by self-induction lags a quarter of a 
phase or 90 behind the current flowing in the circuit. That this 
is so may be seen from the fact that the electromotive force of self- 
induction is a maximum when the current producing it is changing 
most rapidly, and is zero when the current producing it is not 
changing at all. The maximum rate of change of the current flow- 



26 AMERICAN TELEPHONE PRACTICE. 

ing in a circuit occurs when the current is passing through zero, 
and its minimum rate of change occurs at the crests of the wave, 
that is, at its maximum points. It therefore follows that the electro- 
motive force of self-induction is a maximum when the current in 
the circuit is zero, and is zero when the current is a maximum. 
This evidently indicates a phase difference of 90 ° ', and we have 
already seen that this phase difference is a lagging rather than a 
leading one. 

In circuits containing only non-inductive resistance the electro- 
motive force impressed upon the circuit has only to overcome the 
ohmic resistance, and the value of the current may be obtained at 
any time by a direct application of Ohm's law. Where self-induc- 
tion, however, is added, the impressed electromotive force, if it be 
a varying one, must overcome not only the ohmic resistance, but 
the electromotive force of self-induction; and then the current 
equation becomes 

Current = Electromotive Force 
Impedance 

The word impedance in this equation may be termed the apparent 
resistance, and the apparent resistance in circuits having self- 
induction is always greater than the ohmic resistance. In fact, Z, 
the impedance, is equal to 



K^4- 47 rV 2 Z 2 ; 
where / is the frequency of alternations and L is the coefficient of 
self-induction — a term denoting the total number of lines of force 
set up in a given coil when traversed by current of unit strength. 
The equation of the flow of current, I, may then be written 

1-* 



Z t/R 1 + Apt*f V 
which is the equivalent of saying that the current flowing is equal 
to the electromotive force divided by the apparent resistance. 

The current flowing in a circuit in which self-induction and 
resistance are present is the resultant of that produced by the 
impressed electromotive force and the electromotive force of self- 
induction. The greater the electromotive force of self-induction the 
greater will be the lag of the current behind the impressed electro- 
motive force. Furthermore, the greater the self-induction the 
greater w T ill be the apparent resistance or impedance, and conse- 
quently the smaller will be the current flowing. The above formula 
applies only to current varying according to the sine law; but tele- 



INDUCTION AND CAPACITY. 27 

phone currents do not vary according to this law, or according to 
any other definite law, so far as we have been able to determine. 
This does not, however, destroy the significance of the formula as 
applied to telephony. Fourier's theorem states that any complex 
periodic wave motion may be considered as being made up of a 
number of simple wave motions having I, ,2, 3, 4, etc., times the 
rate of vibration of the complex wave motion. Telephone currents 
are very complex, and are composed not only of a fundamental tone, 
but of many overtones; it is by the various blending of these over- 
tones, with regard to their relative loudness and their relative 
position in phase with respect to each other, that articulate speech 
is produced. A consideration of the formula for the flow of current, 
just given, shows that the effect of self-induction is greater upon 
currents of high frequency than upon those of low frequency, for as 
/, the frequency, increases, the value of the impedance or the ap- 
parent resistance increases, and, therefore, the value of the current 
decreases. In other words, self-induction tends to weed out the 
higher overtones in preference to the lower ones and the fundamen- 
tal tone, thus rendering speech indistinct, as well as reducing its 
volume. 

Applying these principles to the practical side 'of telephony, we 
see that the presence of an electro-magnet in the path over which 
voice currents must pass, will be deleterious to good speech trans- 
mission. This is true because the self-induction of the magnet 
tends, as we have seen, to reduce the flow of varying current, and 
to modify the wave form, thus not only reducing the power of the 
transmission, but actually interfering with its quality. 

On the other hand, we may be quite free in connecting electro- 
magnets, having high coefficients of self-induction, in shunt or 
bridging paths across the two sides of a circuit over which voice 
currents must pass, because the very high apparent resistance or 
impedance offered by these magnets, prevents an undue amount of 
the voice currents from being shunted from the path it is desired 
to have them follow. 

The coefficient of self-induction of a coil depends directly on the 
number of turns of wire on it, and also on the number of magnetic 
lines of force set up through the coil by a given current. This latter 
factor depends largely on the form of the core and the amount of 
iron in it, and also on the softness of the iron. To make a magnet 
of high impedance it is therefore necessary to use a comparatively 
large number of turns of wire on it, and to employ a core of soft 



28 AMERICAN TELEPHONE PRACTICE. 

iron of ample cross section, with a return magnetic circuit if 
possible. 

Coming now to the phenomena of capacity: Every insulated 
conductor is capable of receiving a certain charge when subjected 
to an electromotive force ; for instance, if a metallic plate insulated 
from all surrounding bodies is connected to one terminal of a bat- 
tery the other terminal of which is grounded, a certain amount of 
electricity will flow into the plate until its potential is raised to that 
of the battery terminal. The plate is then said to be charged, and 
the amount of electricity held by it determines its capacity. The 
charge of electricity on the plate will be considered positive or nega- 
tive, according to whether the positive or negative terminal of the 
battery, or other charging source, was connected with it. 

It is well known that no charge exists by itself — there is always an 
equal and opposite charge induced by it upon neighboring bodies. 
It is also well known that like charges repel each other, while unlike 
charges attract; that if an uncharged body be brought near a 
charged body an equal and opposite charge will be induced on the 
side of the uncharged body which is toward the charged body, and 
that similarly a charge of the same sign as that on the charged body 
will be induced on the opposite side of the uncharged body. If now 
the body which was originally uncharged is connected with the 
ground, this latter charge — that is, the one of the same sign as the 
original charge — will be driven to the ground, while the charge of 
the opposite sign will still be attracted by the charge on the first 
body. The second body will therefore be charged, although it has 
not been in contact with the first. The action between charges of 
electricity taking place through an insulating medium is called 
electrostatic induction. It is found that where two conductors are 
placed side by side, but insulated from each other, the capacity of 
each will be greater than if the other were not present. For the 
purpose of holding charges in this manner the well-known Leyden 
jars have long been in use. They are usually made by coating a 
glass jar inside and out with a layer of tin-foil to within a few inches 
of the top. The outer coating is usually connected with the ground, 
while the inner coating is connected with a metallic rod approaching 
it through the mouth of the jar. If the inner coating is connected 
Avith a source of electromotive force, a current lasting but an instant 
will flow into the coating, producing a charge. This charge, which 
we will say is positive, will attract a nearly equal negative charge 
to the outer coating, repelling an equal positive charge to the earth, 



INDUCTION AND CAPACITY. 29 

as already described. The amount of charge which the inner coat- 
ing will receive under these circumstances is very much greater 
than if the outer coating were not present, and the capacity of the 
inner coating is therefore much higher than before. Devices con- 
sisting of two electrical conductors separated by an insulating 
medium for the purpose of holding two charges of electricity are 
called condensers. The Leyden jar is, therefore, a type of con- 
denser. 

The capacity of a condenser is increased as the area of the con- 
ducting surface is increased; is increased as the distance between 
the conductors is diminished, and may be increased or diminished 
by using different kinds of insulating material between the conduc- 
tors. The medium separating the conductors is called the dielectric, 
and, as stated above, upon it depends to a great extent the efficiency 
of a condenser. Several condensers built exactly alike, so far as 
size of plates and the distance between them are concerned, and 
using different materials for dielectrics, will be found to have differ- 
ent capacities. This difference is due to- a peculiar property 
possessed to different degrees by different dielectrics and called 
specific inductive capacity. 

The specific inductive capacity of a dielectric is a measure of that 
quality which enables the dielectric to hold a charge between two 
conductors, as in a condenser. The specific inductive capacity of 
air is taken as a standard and is for convenience considered as unity; 
it is lower than that of any other known substance excepting, per- 
haps, hydrogen. If two condensers having plates of equal size and 
distance apart are constructed with dielectrics respectively of air and 
guttapercha, it will be found that the condenser having the dielec- 
tric of guttapercha will receive a charge nearly 2.\ times as great 
as the condenser having the dielectric of air. The actual ratio 
between the two is 2462, and for this reason the specific inductive 
capacity of guttapercha is said to be 2.462. The following table 
gives the specific inductive capacities of some of the more impor- 
tant insulators: 

Air 100 

Glass 3013 

Shellac 2.74 

Sulphur 2.580 

Guttapercha 2.462 

Ebonite 2.284 

India-rubber 2.220 

Paraffin 1 004 

Carbonic Acid 1 .00036 

Hydrogen 0.00007 

Vacuum 0.9994 » 



30 AMERICAN TELEPHONE PRACTICE. 

It is probable that with very rapidly varying electromotive forces, 
such as are dealt with in telephony, the specific inductive capacities 
of the various substances would be higher in comparison with air 
than those indicated by this table. 

Specific inductive capacity is a very important consideration in 
the construction of cables for telephone purposes. In the construc- 
tion of these cables it is desirable, as will be shown later, to reduce 
the capacity of the wires of the cable to as great an extent as pos- 
sible, and in order to do this the dielectric is, in the best forms of 
cables, made to as great an extent as possible of dry air. On the 
other hand, in the construction of condensers it is desired that the 
capacity may be as great as possible for a given area of plates, and 
therefore some material other than air is used. Paper saturated 
with paraffin is perhaps the most commonly used, paraffin having 
about twice as great a specific inductive capacity as air. and more- 
over lending itself readily to the purposes of insulation. To sum 
up. the capacity of a condenser varies in direct proportion as the 
area of its plates, inversely as the square of the distance between 
the plates, and directly as the specific inductive capacity of the 
dielectric. 

Condensers used for telephonic purposes, where a comparatively 
high capacity is desired within a small space, are usually built up 
of alternate layers of tin-foil and paper, and tightly compressed so 
as to bring the plates as close together as possible. The paper is 
usually soaked in paraffin. 

The effect of a condenser bridged across a circuit carrying an 
-alternating current is to absorb a portion of the current as the 
electromotive force at its terminals increases, and as the electron- 
motive force decreases, to give this current back to the line. Con- 
sider such a circuit when the electromotive force active in driving 
current through it begins to rise. The electromotive force at the 
condenser terminals will also rise, and current will therefore flow 
into the condenser. The strength of this current will depend 
directly upon the rate at which the potential at the terminals of the 
condenser is changing. When the electromotive force acting in 
the circuit reaches a maximum, the potential at the condenser ter- 
minals will also be a maximum and will for an instant cease to 
change. At this point the condenser is fully charged, but as the 
electromotive force of the line is not changing, no more current 
flows into the condenser: in other words, the condenser current is 
zero. As the electromotive force in the line decreases, current will 



INDUCTION AND CAPACITY. 31 

flow out of the condenser and into the line, because the condenser 
is not capable of holding so much charge at the lower potential. 
The flow of current out of the condenser reaches a maximum when 
the electromotive force in the line is changing most rapidly, and 
this occurs when it is passing through zero. From this it will be 
seen that the condenser current is zero when the electromotive force 
in the line is a maximum, and is a maximum when the electro- 
motive force in the line is zero. This indicates, as in the case of 
self-induction, a phase of difference of 90 , or a quarter of a cycle. 
It is not so easy to say whether this phase difference is lagging or 
leading, but a consideration of the direction of flow of current 
throughout the cycle will throw some light upon the subject. 

At the instant when the current flowing in the line (which is in 
exact phase with the active electromotive force in the line*) is 
positive and at a maximum, the condenser current will be zero. As 
the active electromotive force decreases toward zero the condenser 
current increases, but in a different direction, — negative, — because 
current is now flowing out of the condenser back to the line. As 
the active electromotive force reaches zero the condenser current 
is at its maximum negative value, and as the active electromotive 
force reaches its maximum negative value the condenser current 
reaches zero. During the next half-cycle the condenser current 
increases to a positive maximum and decreases to zero, while the 
active electromotive force passes from a negative maximum to a 
positive maximum. In other words, while the active electromotive 
force, and therefore the line current with which it is in phase, de- 
creases from a positive maximum value to a negative maximum 
value, the condenser current is negative, and while the active elec- 
tromotive force increases from its negative to its positive maximum 
value the condenser current is positive. The condenser current 
therefore reaches its zero value, while decreasing, 90 in advance 
of the same value of the active electromotive force; its maximum 
negative value 90 in advance of the maximum negative value of 
the active electromotive force; and upon investigation it will be 
found that every value of the condenser current occurs 90 in 
advance of the corresponding value of the actual line current. The 
electromotive force which is in phase with the condenser current is 
called the condenser electromotive force, and is 90° degrees in ad- 



* The active electromotive force is the resultant of the impressed electromotive force 
and the condenser electromotive force, and is in phase with the current actually flowing 
in the line. 



32 AMERICAN TELEPHONE PRACTICE. 

vance of the electromotive force which is active in driving current 
through the line. This latter electromotive force which, as we have 
said, is in phase with the current flowing in the line, is the resultant 
of the impressed electromotive force and the condenser electro- 
motive force, and therefore leads the impressed electromotive force 
by a certain angular distance. 

The current equation for a circuit containing resistance and 
capacity is, as before, 

^ Electromotive Force 

Current = T = . 

Impedance 

In this case the impedance depends on the ohmic resistance of the 

circuit and on its capacity, and is equal to the following expression: 



/ 



R 4- 



where / is the frequency, as before, R the ohmic resistance, and C 
the capacity of the condenser in farads. From this the current 
equation becomes 

E 



/& 



The denominator is the apparent resistance, depending upon the 
ohmic resistance of the circuit, the capacity, and the frequency of 
alternations. An inspection of this equation will show that as the 
frequency, f, is increased the impedance or apparent resistance be- 
comes smaller, and this accounts for the fact that a condenser will 
readily transmit rapidly fluctuating currents, such as voice currents. 
Evidently the effect of increasing / reduces the second member in 
the denominator of the equation, and if sufficiently great, this may 
be neglected, and the equation becomes simply 

R 

Again, increasing the capacity of the condenser also increases 
the effective current by reducing the impedance. 

Considering these facts and formulae from the standpoint of tele- 
phonic practice, it may be seen that a condenser may be introduced 
in the path of voice currents without seriously interfering with their 
passage. Although introducing enormous ohmic resistance into 
the circuit, a good condenser will allow voice currents to pass 
through it by induction to such an extent that its presence cannot 
be detected by one listening to the transmitted sounds. Again, on 



INDUCTION AND CAPACITY. 33 

account of this ability to transmit voice currents, it is very injurious 
to transmission to bridge a condenser of considerable size across the 
two sides of a circuit carrying voice currents, because of the shunt- 
ing effect of the path through the condenser. 

The condenser must be treated in its -relation to telephonic cir- 
cuits in exactly the opposite manner from an, impedance coil. A 
condenser may be put in series without seriously reducing the voice 
currents, while an impedance coil may not. An impedance coil 
may be bridged across the telephonic circuit without evil results, 
while a condenser may not. 

The condenser affords the designer of telephone systems a means 
for allowing varying currents to pass, to the exclusion of unvarying 
currents. An impedance coil may be made a practical bar to the 
passage of rapidly varying currents while allowing steady currents 
to pass freely. 

The overcoming of the deleterious effects of self-induction and 
of capacity on telephonic transmission is the most difficult problem 
with which the telephone engineer has to deal at present. On the 
other hand, these two phenomena often afford the skillful worker 
means for accomplishing results which without their aid would be 
impossible. So it is throughout the whole realm of science and 
invention; phenomena which seem at first to put insurmountable 
difficulties in the way of accomplishing an end, often when better 
understood, finally prove the touchstone of success. 



CHAPTER IV. 
THE TELEPHONE RECEIVER. 

To construct a receiver capable of reproducing speech is a very 
simple matter. In fact, nearly any small electromagnet, with a light 
iron armature, such as is commonly used in electric bells and tele- 
graph instruments, may be made to reproduce, with more or less 
distinctness, sounds uttered in the vicinity of a transmitting appara- 
tus with which it is in circuit. It has proved more difficult, 
however, to construct a receiving instrument which will reproduce 
speech well, and at the same time be practically useful in everyday 
use. 

Aside from actual talking efficiency, many considerations of a 
purely mechanical nature enter into the design of a good telephone 
receiver. It should be durable and capable of withstanding the 
rough usage to which it will necessarily be subjected by careless or 
ignorant users. It should be of such construction that its adjust- 
ment will not be changed by mechanical shocks or by changes in 
temperature. Failure to provide against this latter effect is one of 
the chief sources of trouble in telephone work. It should be of such 
external configuration as to enable it to be conveniently placed to 
the ear. The chamber in which the diaphragm vibrates should be 
small and of such shape as not to muffle the sound. The binding 
posts should be so securely fastened in as to prevent their becoming 
loose and twisting off the wires inside the receiver shell; and the 
construction should be so simple as to render the replacing of any 
damaged part an easy matter. 

A few years ago most of the receivers used in America were of 
the single-pole type ; but nearly all of these have now been replaced 
by those of the bipolar type. The particular form shown in Fig. 22 
proved effective, and was formerly almost universally used by the 
American Bell Telephone Company. Its chief merit lies in its 
simplicity. 

In this figure, M is a compound bar-magnet, composed of two 
pairs of separately magnetized steel bars arranged with like poles 
together. Between the pairs of bars is clamped a soft-iron pole- 
piece, P, at one end, and a similarly shaped iron block, 0, at the 

34 



THE TELEPHONE RECEIVER. 



35 



other end. These parts are firmly bound together by the two 
screws, S S. On the end of the pole-piece is slipped a coil of wire, 
G. This coil is usually wound with two parallel No. 38 B. & S. 
silk-insulated copper wires, having a resistance in multiple of about 
75 ohms. 

The magnet is encased in a shell of hard rubber, composed of two 
pieces, A and B, which screw together and clamp between them 




FIG. 22.— BELL SINGLE-POLE RECEIVER. 

the diaphragm, D, of thin sheet iron. The piece, B, is hollowed 
out as shown, to form a convenient earpiece. A hard rubber tail- 
piece, T, carrying two binding posts, //, fits over the end of the 
case opposite the earpiece, B, and is held in place by a screw. E. 
This screw engages a threaded hole in the block, Q, and serves not 
only to hold the tailpiece in place, but to bind the magnet securely 
to the shell. Soldered to the binding posts are heavy leading-in 
wires, W W, which pass along the sides of the magnet and are 
soldered to the respective terminals of the fine wire forming the coil. 



36 



AMERICAN TELEPHONE PRACTICE. 



The diaphragm of this instrument is about i-ioo" in thickness 
and 2\" in diameter. The diameter of the free portion is if". 

Fig. 23 shows the external appearance of this instrument. In 
some single-pole receivers a magnet, consisting of a single cylindri- 
cal bar of steel was used instead of the compound magnet formed 
of several separately magnetized bars, but with generally inferior 
results, owing to its weaker and less permanent magnetic field. 

In many forms of receiving instruments much trouble is experi- 
enced in keeping permanent the adjustment between the magnet 
and the diaphragm. This is often due to flimsy construction, but 
in many of the old designs to the fact that steel and hard rubber 
differ widely as to their amounts of expansion or contraction under 
changes in temperature. In instruments where the magnet is 




FIG. 23.— BELL SIXGLE-POLE RECEIVER. 



rigidly secured to the shell only at a point at considerable distance 
from the diaphragm, the unequal expansion or contraction of the 
magnet and the shell causes the distance between the pole-piece 
and the diaphragm to vary with every change in temperature. A 
sudden change will thus often render a receiver inoperative. 

This defect is seen to exist without any attempt at a remedy in 
the receiver shown in Fig. 22. The point of support of the magnet 
is as far removed from the diaphragm as possible, being at the 
screw. E, and therefore the full effect of all the differences in con- 
traction and expansion between the hard rubber and the steel is 
obtained. 

In bipolar receivers which have now come into almost universal 
use, the object is to strengthen the field in which the diaphragm 
vibrates, by presenting both magnet poles to the diaphragm. The 
length of the path of the lines of force through the air is thus greatly 



THE TELEPHONE RECEIVER. 



37 



shortened, and the field of force is concentrated at the point where 
it will be most effective. 

It is important to have faulty construction pointed out, and the 
bipolar receiver shown in Fig. 24, which unfortunately came into 
wide use, may be cited historically as a "horrible example." 

The shell, A, and earpiece, B, were of a material resembling hard 
rubber, and clamped between them the soft-iron diaphragm, D, as 
in the single-pole instrument described. 




FIG. 24.-OLD TYPE BIPOLAR RECEIVER. 



The magnet consisted of two pairs of separately magnetized steel 
bars, F F, the bars in each pair being laid with like poles together, 
thus forming two compound bar-magnets. These were so laid to- 
gether that the north pole of one pair was opposite the south pole 
of the other. The two pairs of bars were held apart at one end by 
the adjustment block H, made of the same material as the shell, and 
at the other end by the soft-iron block. /. On each side of the 



38 AMERICAN TELEPHONE PRACTICE. 

block, H, and between it and the pairs of bar-magnets, were the soft- 
iron pole-pieces, P P, on which were wound the coils, G G, having 
a resistance of about 50 ohms each. These coils were connected 
in series so that the total resistance of the receiver was 100 ohms. 

The block, H, had two segmental flanges, screw-threaded on their 
circumferential surfaces so as to engage a thread, g, on the inner 
surface of the shell, A. The magnet could thus be adjusted toward 
or from the diaphragm by turning it in the shell, A. 

A tailpiece, T, of imitation hard rubber was shouldered to fit into 
the small end of the receiver shell, and a screw, E, extending 
through it into the block, /, served to clamp the magnet in position. 
To the binding posts, / /, were soldered heavy leading-in wires, 
W W, which passed through holes in the adjustment block, H f and 
were soldered to the terminals of the fine magnet wire. 

In this receiver an attempt was made to remedy the defect caused 
by the unequal contraction and expansion of the parts by securing 
the magnet to the shell at a point close to the diaphragm, thus re- 
ducing the differences in expansion and contraction to a minimum. 
In this particular case, however, this introduced a defect quite as 
serious, because the shell was also bound to the other end of the 
magnet by the screw, E. The contraction and expansion thus 
tended to loosen the screw-thread on block, H, making frequent 
readjustment necessary. Moreover, a good screw-driver in the 
hands of an ordinary repair man, or of a subscriber, often subjected 
the screw-thread on block, H, to such a strain as to strip it, thus 
rendering the receiver useless. 

Several important points may be learned from the behavior of the 
two forms of receivers shown in Figs. 22 and 24. 

It indicates very faulty design to secure the' magnet in the shell 
at the farthest end of the diaphragm. It is equally bad to fasten the 
magnet and shell rigidly together at both ends, unless the shell is 
of metal. The use of any of the old materials in imitation of hard 
rubber has proven attractive but disastrous. Attractive on account 
of its low cost and the facility with which it could be moulded. 
Disastrous because, until a very recent date, at least, these materials 
were usually subject to some or all of the following faults to a 
greater extent than hard rubber: They were very brittle and liable 
to have "cold shuts" or seams formed in moulding, which resulted 
in a lack of toughness and liability to cause cracks or fractures. 
They were often hygroscopic, tending to absorb moisture and thus 
reduce their insulating properties. Some of them softened under 



THE TELEPHONE RECEIVER. 



W 



heat and gave way slowly under long continued pressure to an even 
greater extent than hard rubber, this resulting in permanently dis- 
torted forms when cooled or relieved from pressure. Lastly, many 
of these materials would discolor and roughen with use, thus spoil- 
ing their appearance and rendering them unpleasant to the touch. 

Recent improvements in these materials have been made, how- 
ever, which seem to justify the hope that a material as good as hard 
rubber may be found. Composition shells are again being used to 
a limited extent by reliable manufacturers, as in some of the new 
compositions little seems lacking in the matter of toughness, in- 
sulating qualities and ability to resist an ordinary amount of 




FIG. 25.— ADER BIPOLAR RECEIVER. 

moisture. Moreover, the improved material does not seem to 
possess the objectionable softening and changing under heat and 
pressure to as great an extent as hard rubber. 

A way of obviating the expansion and contraction difficulty used 
largely in European countries and to some extent in this country, is 
to construct the shell holding the diaphragm of some such metal 
as to give it nearly the same coefficient of expansion as the steel 



Fig, 25 shows one of the early forms of bipolar receivers. This 



40 



AMERICAN TELEPHONE PRACTICE. 



was devised in 1881 by Clement Ader. of Paris, France, and is with 
some modifications still used to a limited extent in Europe. This 
embodies an interesting and ingenious attempt at increasing the 
electrical efficiency of the telephone receiver. The magnet, B, is 
ring-shaped, and has fastened to its poles two L-shaped pole-pieces 
carrying coils, C C. The box, R, inclosing the pole-pieces and coils 
is of brass and is secured to the magnet by screws, E E. It is screw- 
threaded at G, so as to engage a corresponding screw-thread on the 
inner surface of the cap, S\ which has a flaring portion, H, forming 
an ear-piece. The diaphragm, D, is clamped between the pieces, 
R and 5, in the usual manner. 

Surrounding the opening, leading from the diaphragm to the ear- 



4= 



£3 




FIG. 26.— WESTERN ELECTRIC RECEIVER. 



piece, is a ring, m, of soft iron, and in this ring lies the chief point 
of Ader's invention. The additional mass of iron placed near the 
poles of the magnet affords a more ready path for the lines of force, 
and their number is thus increased. The diaphragm, therefore, 
moves in a stronger field of force, and the power of the receiver is 
said to be correspondingly augmented. Practice in this country 
has not, however, shown any perceptible gain of efficiency by the 
use of this ring. 



THE TELEPHONE RECEIVER. 



41 



Fig. 26 shows the latest form of receiver used by the Bell licensees 
companies and manufactured by the Western Electric Company of 
Chicago. 

The working parts of this receiver are composed of two magnets, 
A A', clamped together by screws passing through an iron tail- 
block, B, at one end, and through a threaded block, C, of brass at 
the end nearest the magnet coils. The pole-pieces, p p' ', are clamped 
between the pole end of the magnets and the block C, and carry on 
their outer ends the coils c c'. Threaded block C engages a screw- 
threaded portion on the interior of the body of the shell, and by 
turning the block in this threaded portion, adjustment of the mag- 




FIG. 27.— KELLOGG RECEIVER. 



net poles toward or from the diaphragm may be readily accom- 
plished. When this adjustment is once determined, however, in 
the factory, it is made permanent by driving a pin through a flange 
on the block and into the hard rubber, thus preventing accidental 
derangement of the parts. The diaphragm is clamped between the 
body of the shell and the ear-piece in the usual manner. The shell 
and ear-piece of this receiver are made of the best possible grade of 
hard rubber, carefully turned and finished. The binding posts. E, 
are of the lock-nut type and are provided with flanges, c. by means 
of which, and flat-headed machine screws, they are firmly secured 
to the rubber shell. An eyelet, F. also screws into the end of the 



42 



AM ERIC AX TELEPHOXE PRACTICE. 



shell, serving as a means for attaching the straining cord of the 
receiver cord so as to prevent any strain (such as might be caused 
by dropping the receiver) from coming on the cord tips themselves. 

In order to give this receiver sufficient weight to have it properly 
operate the automatic hook switches in ordinary use, a lead weight, 
g, is clamped between the bars, A A', of the magnets. The total 
weight of this receiver (and in nearly all others of its general type) 
is about fifteen (15) ounces. 

In Fig. 2j the latest receiver of the Kellogg Switchboard & 
Supply Company is shown in section. This resembles in its main 
features the receiver of the Western Electric Company, just de- 
scribed, but in addition possesses the very desirable feature of 
having no exposed metallic parts on the exterior of the shell. 
Instead of having the binding posts on the outside of the receiver, 
the cord passes directly through a hole in the small end of the shell, 
its two strands terminating in flat cord tips which may be attached 
by means of screws to the clips, a a\ secured on an insulating block 




FIG. 2?.— RECEIVER WITH XO EXPOSED METALLIC TARTS. 



carried between the magnets. The terminals of the coils are per- 
manently secured to these clips, thus completing the circuit of the 
receiver. The straining cord is tied around the iron block, b, be- 
tween the ends of the magnets. Instead of securing the magnets 
within the shell by means of a screw-threaded block, as in 
case of the Western Electric receiver, a flanged block, c, is pro- 
vided in the Kellogg receiver, which rests on a shoulder on the 
interior of the shell and is secured thereto by means of ordinary 
machine screws. The shell and cap of this receiver are usually 
made of hard rubber, turned and polished, but the Kellogg Com- 
pany has recently also put out a composition shell which seems to 
possess merit while at the same time considerably reducing the cost 
of the receiver. 



THE TELEPHONE RECEIVER. 



4a 



This receiver has been adversely criticised on account of the fact 
that it is necessary to remove the magnets from the shell in order 
to put in a new cord. This objection, however, is probably fully 
offset by the fact that the cord tips, which are always the weakest 
portion of a cord, are concealed from the user and are protected 
from any possible mechanical injury. 

An external view of this type of receiver, with no exposed metal 
parts, is shown in Fig. 28. 

In Fig. 29 the form of receiver originally introduced by the 
Western Telephone Construction Company of Chicago is shown. 
This was the first receiver of the type having no exposed metal 
surfaces, and this particular design has been widely copied. The 




FIG. 29.— WESTERN TELEPHONE CONSTRUCTION COMPANY'S RECEIVER. 



magnet is of horse-shoe form, consisting of a single bar of magnet 
steel bent into the form shown. On the forward ends of the mag- 
net is carried a block of brass grooved on each side to partially 
enclose the magnet limbs. The rear portion of this block is screw- 
threaded, as shown, so as to engage the corresponding thread 
turned in the interior surface of the shell in a manner similar to 
the design shown in Fig. 26. The unique point in this receiver is 
in the method of attaching the receiver cord to the terminals lead- 
ing from the coils. These terminals are composed of heavy, 
insulated wire, passing through the brass block to the coil chamber; 
the other ends of these wires are twisted together and passed 
through a central opening in the rear of the shell, where each is 
soldered to a connector held in place against the shell by a small 
machine screw. The cord is provided with similar connectors 



44 



AMERICAN TELEPHONE PRACTICE. 



which may be slipped under the screw heads, thus completing the 
circuit between the cord and the coils of the receiver. After these 
connections are made a tail-cap composed of hard rubber or com- 
position is screwed in place as shown, thus completely covering the 
cord connectors. An enlargement in the covering of the cord 
effectually prevents any strain coming on the cord terminals when 
the receiver is dropped. 

The receivers shown in Figs. 26 and 27 are representative of 
the highest type of receiver construction in America to-day. They 
are extremely efficient and durable. The design shown in Fig. 29 
is also good, but this particular form has suffered from poor 




C C C 

FIG. 30.— STROMBERG-CARLSON RECEIVER. 



workmanship in manufacture. The tendency of American prac- 
tice seems to be at present toward this general form of receiver, 
with binding posts concealed. 

The Stromberg-Carlson receiver is a very powerful one and has 
stood the test of time well. It is shown in Fig. 30, in which A is 
a casing of brass forming a framework upon which all of the other 
parts of the instrument are supported. This is screw-threaded on 
its outer surface to receive the internally screw-threaded cap. B, 



THE TELEPHONE RECEIVER. 45 

and the lock ring, B' s . One unique feature of this receiver is the 
method of supporting the diaphragm, which is held in place in the 
cap B by the clamping ring B'. 

Upon the cap, B, is screwed an ear-piece, B 2 , of hard rubber. 
The lock-ring, B'\ is adapted to be screwed against the cap, to lock 
it in any adjusted position on the casing A.- Upon the rear of the 
casing is provided a projection, A', against the faces of which rest 
the soft-iron cores, C and C 2 , which extend through the bottom of 
the casing and carry upon their end the coils, C 3 C\ The ends of 
the permanent magnet, D D' , rest upon the cores, C and C 2 , and a 
screw or bolt, £, passes through the ends of the magnet, the cores, 
and the projection, to maintain them in position. The ends of the 
magnet, d, are cut away as shown to permit the cores to be set 
flush with the inner faces of the magnet. 

Between the limbs of the magnet is provided a block, D, of fiber 
upon which are mounted two binding posts, D 2 , these being con- 
nected to the terminals of the coils by heavy insulated wires, D 4 Z) 5 . 




FIG. 32.-ERICSSON RECEIVER. 



To the binding posts are also attached the ends of the receiver cord. 
Upon the rear of the casing, A, is provided a threaded flange upon 
which the insulated casing, or rubber shell, F, is screwed, this latter 
being provided with- an opening at the end through which the re- 
ceiver cord passes. 

The magnet is mounted rigidly upon the metal casing. A, the 
rubber shell being entirely independent so that it may be removed 
by unscrewing. The diaphragm support or cap, B, may be raised 
or lowered to adjust the diaphragm relatively to the magnet cores. 
the ring, B s , serving to lock the diaphragm in its adjusted position. 

This receiver does away entirely with the troublesome effects due 
to expansion or contraction. The insulating casing forms a handle 
and serves as a protection to the cord terminals, but forms no part 
of the working structure itself. 

Still another form of receiver, and one of the non-adjustable tvpe. 
is shown in Fig. 32; this is manufactured by the Ericsson Company, 
of Sweden, and is being imported into this country to a considerable 



46 AMERICAN TELEPHONE PRACTICE. 

extent. This is of the bipolar type. The magnets are secured to 
the metal cup by means of two screws shown in the figure, each 
extending transversely through the case and into the magnets. 
The holes in the case through which these screws project are 
slotted so that a certain amount of adjustment can be obtained if 
it is absolutely necessary, although the idea of the manufacturers 
is to bind it so tightly that no adjustment will ever be needed. 




FIG. 33.— MURDOCK SOLID RECEIVER. 

The inclosing tube for the magnets is of brass covered by a thin 
layer of insulating material, usually hard rubber, but sometimes of 
leather. This tube is also held in position by the screws before 
mentioned. A piece of hard rubber projects between the two bind- 
ing posts of the instrument as shown, the object of this being to 
prevent the tips of the receiver cords from twisting the posts in 
their sockets until they touch each other, thus short-circuiting the 
instrument. This receiver is well made, handsome, and efficient. 
The receiver shown in Fig. 33, manufactured by W. J. Murdock, 



THE TELEPHONE RECEIVER. 



47 



Chelsea, Mass., embodies a decidedly novel feature. In this all the 
working parts of the receiver, including the permanent horse-shoe 
magnet, the pole-pieces, the binding posts and the leading-in wires, 
are moulded in a shell so as to form one integral part. The dia- 
phragm is held in place by the ordinary ear-piece, which screws in 
place as in other receivers. The coils slip bn over the pole-pieces, 
this being the only way of replacing a coil in case of a burn-out or 
injury without destroying the shell itself. It has now been on the 
market for over a year and is apparently giving satisfaction. 




FIG. 34,— PARTS OF WATCH-CASE RECEIVER. 



The diaphragms used for receivers are made of very soft thin 
sheet iron; the ferrotype plate formerly used for tin-types in pho- 
tography being as good material as can be found for this purpose. 
Some companies, however, are using tinned-iron diaphragms, which 
give equally good results. 

The diaphragms for the various receivers here described vary 
from 2 to 2\ inches in diameter, the free portions — that is, the por- 
tions not clamped by the supports — ranging from i \ to 2 inches. 
The usual thickness is from .009 to .on of an inch. The thickness 



43 AMERICAN TELEFHOXE PRACTICE. 

of a diaphragm, to produce the best results with a given receiver, 
must be obtained by experiment, as it depends on the diameter of 
the portion free to vibrate, and also on the strength of the magnetic 
field, due to the permanent magnet. It has been shown that with 
a very thin diaphragm and a very powerful magnet the iron in the 
diaphragm becomes saturated, so that it is not responsive to changes 
in the strength of the existing field. 

In certain classes of telephone work it becomes desirable or 
necessary to have the receiver held constantly to the ear of the user. 
This is true particularly in the case of the work of a switchboard 
operator, who must have both hands free for manipulating the 
switchboard. Any of the types of receiver so far described would 
prove too cumbersome and heavy for this use, and therefore a differ- 
ent type possessing small weight and volume has come into ex- 
istence. These are commonly known as "watch-case" or '"head" 
receivers. 

In Fig. 34 are shown the parts 01 such a receiver. The per- 
manent magnets are of the ring type, these being cross -mag- 
netized so as to produce poles on opposite sides of their circum- 
ferences. Secured to the compound ring magnet are circular pole- 
pieces which carry the coils, these pole-pieces presenting their 
pole-faces to the diaphragm, as in the ordinary types of hand 
receivers. The magnet and coils are mounted in a hard rubber cup 
or shell, and the diaphragm is clamped between the front face of 
this cup and the earpiece in the usual manner. The binding posts 
to which the terminals of the cord are attached are usually placed 
within the shell, this being particularly desirable in this type of 
receiver because of the liability, if left outside, of entangling the 
operator's hair. Such receivers, if properly designed and con- 
structed, may be made to possess almost as great an efficiency as 
the best hand receivers, while their weight is only a few ounces, and 
their form so compact as to enable them to be worn without dis- 
comfort to the operators. 

In Fig. 35 is shown such a receiver assembled, with a head band 
for securing it to the ear of the operator. This head band is a light 
strip of flexible steel, covered with leather. It is attached to the 
receiver by a hinge joint as shown, in order to adapt itself more 
readily to the ear of the wearer. Many different forms of head 
bands have come into use. but the type shown, consisting of but a 
single band, is coming into most favor. A good operator's receiver 
and head band should, while being efficient, be light and compact, 



THE TELEPHONE RECEIVER. 



49 



and be so constructed as to avoid catching the operator's hair. In 
order to prevent possible electric shocks to the operator all exposed 
metallic parts should be carefully insulated from the coils and other 
parts having electrical functions. 

The question of receiver cords is one of a good deal of importance, 
as a faulty cord is one of the most prolific sources of trouble in any 
part of a telephone instrument. If the conductors in a cord are not 
properly insulated, so that they may come into contact with each 
other, or if a break occurs in one of the conductors, the instrument 
will be short-circuited in the one case or left open in the other. The 




FIG. 



-WATCH-CASE RECEIVER WITH HEADBAND. 



conductors in the best receiver cords are usually composed of tinsel, 
laid up in the manner of ordinary twisted rope ; a few strands of 
fine copper wire are frequently added to give greater strength. These 
tinsel conductors are then tightly wrapped with silk and given one 
or more braids, usually of cotton, after which the two conductors so 
insulated are laid together and braided over with a worsted or silk 



50 AMERICAN TELEPHONE PRACTICE. 

braid. Frequently a spiral wrapping of spring brass wire is put 
about the two insulated conductors before the outer braid is put on. 

Cord tips are necessarily subject to a rather rough usage, as it fre- 
quently happens that a receiver is dropped, thus allowing a heavy 
strain to come on the cords which is usually most severe just where 
the tip joins the cord proper. 

The method of fastening the tip to the cord, shown in Fig. 36, has 




FIG. 36.— DETAILS OF RECEIVER CORD TIP. 

become very popular. In this, a needle or pin B of No. 14 brass wire 
tapered to a long point, is inserted into the hollow of a braided tinsel 
cord for about half an inch, when the end is passed out through the 
conductor and covering and bent backwards, forming a hook, as 
shown at D and A. Before the pin is put in, the conductor is bared 
for a short distance, and after the pin is inserted it is wound with a 
fine wire and soldered. The tip is then finished with a spiral of wire, 
as shown at E, or w T ith a shell, as shown at C, the latter being most 
common. 

The cord tip used by the Kellogg Switchboard and Supply Com- 
pany is shown in Fig. 37. In this the tinsel conductor is looped back 
on itself and wound with a linen thread, this thread extending back 



FIG. 37.— KELLOGG CORD TIP. 

over the silk or worsted braiding, as shown at A. A collar of sheet 
brass is then formed around the portion thus served, this collar being 
shown in process of formation at B, and in its completed form at 
C. The cord tip proper is composed of a piece of brass rod bored 
out at one end, a, to receive the end of the cord, and turned down at 
the other end, b, to fit an ordinary binding post. The prepared end of 
the cord, as shown at C, is secured in the hollow portion of the cord 



THE TELEPHONE RECEIVER. 



51 



tip by means of solder, the completed tip being- shown at D. By this 
construction the strain of the tip is borne by the combined strength 
of the braid and of the conductor, and electrical contact between the 
tinsel and the tip proper is assured by means of the brass collar and 
the solder. 

The cord tip of the Western Electric Company is shown in Fig. 



FIG. 38.— WESTERN ELECTRIC CORD TIP. 

38. In this the tinsel is first bared for a distance of about half an 
inch, and then the braid is passed back for about an equal distance 
farther, while a wrapping of brass wire is put tightly on the tinsel. 
The braiding is then partially returned over this wrapping of wire, 
and is served with linen thread, as shown at A, after which the end 
thus prepared is soldered in the hollow end of the tip proper, as 
shown at B. 

In order to prevent an undue strain on the tips when a receiver 
is dropped, it is best to have the cord provided at each end with an 




FIG. 



-SUPPORTING LOOP AND HOOK FOR RECEIVER CORD. 



auxiliary string firmly secured to the braiding or outer covering of 
the cord, which may be tied to the receiver at one end, and to some 
support on the telephone at the other, in such manner as to take up 
the strain due to the weight of the receiver, rather than letting this 
. rain come upon the conductors and tips themselves. This strain- 
ins: stringf, as it is usually called, is, as a rule, a continuation of 



52 AMERICAN TELEPHONE PRACTICE. 

the outer braiding of the cord, and may be fastened to an eyelet in 
the receiver or to a small link on one of the binding posts, as shown 
in the left-hand portion of Fig. 39. Another way of accomplishing 
the same result is by means of a small wire hook sewed to the braid- 
ing, just at the fork of the cord, which hook may be closed by pliers 
around a screw-eye, as shown in the right-hand portion of Fig. 39. 

Any receiver which is not provided with some means of taking the 
strain off the cord tips or cord fastenings, should be considered 
faulty, as the conductors and cord tips are frail at best, and should 
not be subjected to the strain which would otherwise be put upon 
them by the careless user. The receiver of the Western Electric 
Company, shown in Fig. 26, uses a straining string attached to an 
eyelet, as above described. In the receiver of the Kellogg Company 
(Fig. 27), with its internal binding posts, the straining cord is tied 
around the block at the end of the magnet, which is the better prac- 
tice, because the strain is thus taken directly by the heavy part of 
the receiver without the intervention of the rubber shell. The re- 
ceiver of the Stromberg-Carlson Company (Fig. 30) has this same 
feature. 



CHAPTER V. 



THE CARBON TRANSMITTER. 



Many vain attempts have been made to discover a satisfactory 
substitute for carbon as the variable resistance medium in tele- 
phone transmitters, the patents on the use of carbon electrodes 
having, at first, formed one of the mainstays of the American Bell 
Telephone Company's great monopoly. 

The theory of the action of carbon in the transmitter has been 
the subject of much discussion. As previously pointed out, any 
motion of the diaphragm which increases the pressure between the 
electrodes lowers the resistance between them, thus allowing the 
passage of a greater current. A decrease of pressure produces 
the opposite result. 

Four different explanations for this action have been put forth, 
and are as follows : 

First, that the electrical resistance of the carbon itself is caused 
to vary by the changes in pressure. 

Second, that a film of air or gas exists between the electrodes, 
and that the thickness of this film is varied by the changes in 
pressure, thus varying the resistance. This theory is apparently 
still adhered to by Mr. Berliner.* 

Third, that the peculiar property possessed by carbon of lower- 
ing its resistance with increased temperature is in the following 
way accountable for the action, in part at least : that an increase of 
current (due to increased pressure and diminished resistance be- 
tween the electrodes) causes a slight heating at the point of con- 
tact ; that this heating causes a still further diminution of resistance 
with an additional increase of current ; and that conversely a mo- 
mentary decrease of current causes a decrease of temperature with 
a corresponding additional increase of resistance and diminution 
of current. 

Fourth, that change in resistance is due to the variation in the 
area of contact between the electrodes — that is. the variation in the 
number of molecules in actual contact. This change in area is 
perfectly apparent in the liquid transmitter of Bell, and in. the case 



"Microphonic Telephonic Action," by Emile Berliner. American Electrician, March. ISf 7. 



54 AMERICAN TELEPHONE PRACTICE. 

of solid electrodes may be well illustrated by the following well- 
known experiment : 

If a billiard ball be gently pressed on a plain marble slab coated 
with graphite, the area of contact of the ball with the slab will be 
indicated by a small dot of graphite on the ball. If, now, the ball 
be dropped from a considerable height, it will be noticed that the 
spot of graphite on the ball is much larger, showing that the ball 
has flattened out to a considerable extent, owing to the greater 
pressure exerted. This demonstrates clearly the variation in area 
of contact between two bodies, due to variations of pressure be- 
tween them. Of course, if the two bodies are conductors of elec- 
tricity, the resistance between them will vary inversely and the 
current directly as the area of contact. 

It seems most probable that of the above explanations, the fourth 
is the true one, and that none of the others aid in any perceptible 
degree in producing desirable effects in the microphone. 

As to the first explanation, that the resistance of the carbon itself 
changes under pressure, experiments have been made with long 
carbon rods; and with measuring instruments of ordinary sensi- 
bility no difference whatever could be detected in the resistance of 
a rod when the pressure on it was varied from zero up to the crush- 
ing point, care being taken that none oHhe contacts in circuit were 
subjected to the change in pressure. 

As to the layer of air theory, Professor Fessenden has thrown 
some light upon it,* by showing that if the layer of air were in the 
ordinary gaseous state, its resistance would be almost infinite, while 
if it existed in some peculiar condensed state of which we know 
little, but in which air might be conceived to be a conductor, then 
the law of change of resistance between the electrodes would be 
different from what it has actually been found to be. On the other 
hand, the curves plotted with resistances as ordinates and with dis- 
tances as abscissas have been found by Professor Fessenden and by 
Messrs. Ross and Dougherty to exactly agree with the form ob- 
tained from theoretical considerations on the basis that the change 
in resistance is due to area of surface contact alone. 

As to the third explanation, it may be said that the very fact that 
the increase of current is needed to cause the rise of temperature 
seems to preclude the supposition that the rise of temperature 
should cause the diminution of resistance with its corresponding 

* " Microphonic Telephonic Action," by Professor R. A. Fessenden, American Electrician, 
May, 1897. 



CARBON TRANSMITTER. 



55 



increase of current in time to do any good. The heating effects in 
carbon are comparatively slow, and it would seem that the changes 
in temperature would lag slightly behind the changes in current 
producing them, in such a manner as to be detrimental to telephone 
transmission. 

It is certainly most fortunate that in one' substance — carbon — 
should be found all the qualifications which make it particularly 
desirable for microphonic work. It produces the change in re- 




FIG. 40.-THE BLAKE TRANSMITTER. 



sistance with changes in surface contact, all things considered, 
better than any other known substance, possesses the desirable 
property of lowering its resistance when heated, and is elastic, non- 
corrosive, non-fusible, cheap, and easily worked. 

The form of transmitter almost universally used in this country 
up to within a few years ago, but now almost obsolete, is that de- 
vised by Francis Blake of Boston. This instrument is shown in 
Fig. 40, in which B represents a metal ring or frame for holding 



56 AMERICAN TELEPHOXE PRACTICE. 

the mechanism of the instrument. It is. screwed to the cover, A\ 
of the box A, and has two diametrically opposed lugs, B' B 2 . On 
this ring is mounted the diaphragm, C, of rather heavy sheet- 
iron, supported in a rubber ring, r, stretched around its edge, and 
is held in place by two damping springs, D D, each bearing on a 
small block of soft rubber, a, resting on the diaphragm at a point 
near its center. The object of these damping springs is to pre- 
vent too great an amplitude of vibration of the diaphragm, and 
also to keep it from vibrating in separate parts instead of as a 
unit. 

Opposite the center of the diaphragm is the orifice, E, in the 
cover, A', so hollowed out as to form a mouthpiece. The adjusting 
lever, F, is attached to the spring, ;', secured to the lug, B' ', of 
the ring, B. The lower end of this lever rests upon an adjusting 
screw, G, in the lug, B 2 , which is drilled and slotted as shown to 
prevent the screw from working loose. On the back of the dia- 
phragm and at its center is placed the front electrode, consisting 
of a small bar, e, of platinum ; one end of the bar rests against 
the diaphragm, while the other end is brought to a blunt point 
and is in contact with the back electrode, c' . The electrode, e, is 
supported independently upon a light spring, c, mounted on the 
lever, F, but insulated from it. This spring tends to press away 
from the diaphragm and toward the back electrode. The back elec- 
trode is formed of a block of carbon, c' , set into a brass block, 
g, of considerable weight, mounted on a spring, d, supported on 
the adjusting lever, F. This spring, d, has a tension in the oppo- 
site direction to that of the spring, c, and being stronger than the 
latter it keeps the electrode, e, in contact with the diaphragm. 

It is seen that instead of having one of the electrodes held in 
fixed position while the other is pressed against it with greater or 
less force by the vibration of the diaphragm with which it is con- 
nected, both electrodes are supported in such manner as to be ca- 
pable of moving with the diaphragm, but the outer electrode is so 
weighted that its inertia will offer enough resistance to the slight and 
rapid vibrations of the diaphragm to give a varying pressure be- 
tween the electrodes and consequent changes of the resistance of 
the circuit. By this means the initial pressure between the two elec- 
trodes will not be affected by changes of temperature, and the 
adjustment will therefore be more nearly permanent. 

This transmitter is very delicate, and transmits the quality of 
the voice in a manner unexcelled by others. It is, however, lack- 



CARBON TRANSMITTER. 57 

ing in power, especially when compared with instruments of later 
design. Besides this, it has a tendency to rattle or break contact 
when acted on by loud noises. 

Fig. 41 illustrates the Crossley transmitter, introduced into Europe 
early in 1879. This well illustrates the class very appropriately 
termed "multiple-electrode" transmitters. Transmitters devised by 
Johnson, Gower, Ader, D'Arsoncal, Turnbull, and many others are 
of this type, and differ merely in the arrangement and number of 
electrodes. They give, as a rule, more powerful results than the 
transmitters having a single pair of electrodes, but most of them are 
subject to the grave defect of breaking the circuit entirely when sub- 
jected to loud noises. 

In this figure, / represents a diaphragm formed of a thin piece 
of pine board about -J" thick and mounted on a supporting ring, K. 
Fastened to this diaphragm are four carbon blocks, F, G, H and / in 




I 

FIG. 41.— THE CROSSLEY TRANSMITTER. 

the relative positions shown. These are hollowed out to receive 
the conical ends of the carbon pencils, A, B, C and D, which are 
supported loosely between them. The blocks, H and G, form the 
terminals of the transmitter. The current divides at the block, H, 
and passes through the pencils, A and C, in multiple to the blocks. 
F and /, and thence through the pencils, B and D, to the other elec- 
trode, G. Vibrations of the diaphragm cause variations in the 
intimacy of contact between the eight points of support of the four 
rods, and thus produce the desired fluctuations in resistance. It is 
seen that this is merely a modification of the Hughes microphone, 
the principles being the same, but the multiple contact allows a 
greater current to pass through the transmitter, and at the same 
time produce greater changes in this current than in the original 
form, where a single pencil was used. Moreover, the liability of 
"rattling" is greatly reduced. 

Fig. 42 shows the Turnbull transmitter, which has been used to 



58 



AMERICAN TELEPHONE PRACTICE. 



a limited extent in this country, even until recently. In this figure, 
A is the diaphragm of thin wood, on the back of which is mounted 
the bracket, B. Pivoted on a rod, b, carried by this bracket, are 
several carbon rods or pendants, a, which rest at their lower end 
against a carbon rod, c, carried on a bracket, C, also mounted on 




FIG. 42.— THE TURNBULL TRANSMITTER. 

the diaphragm. The rods, b and C, form the terminals of the trans- 
mitter, and the current passes from one of them through the carbon 
pendants in multiple to the other. The variations in resistance 
occur principally in the contacts between the rod, C, and the pen- 
dants, a, although there is an additional effect between the pendants 
and the supporting rod, b, particularly if this rod is made of carbon. 




FIG. 43.— THE CLAMOND TRANSMITTER. 



Fig. 43 shows still another form of the multiple-electrode trans- 
mitter, using carbon balls instead of pencils or pendants. A 
represents the vibratory diaphragm of carbon; B a plate of carbon 
having a number of cylindrical cavities, 1 1, upon one side. Fitting 
loosely in each cavity is a ball of carbon, s. The depth of the cavi- 



CARBON TRANSMITTER. 



59 



ties is a little less than half the diameter of the balls, and the 
diaphragm is so placed in the front of the plate that the balls, follow- 
ing their tendency to roll out of the cavities, will rest against its 
inner surface and also upon the edges of the cavities. Many other 
forms of instruments have been devised using one or more balls 
held in various positions between carbon plates. Few are used 
to-day, and all the transmitters so far described are being rapidly 
replaced by the Hunnings form of instrument, which, as has already 
been stated, uses granules of carbon for the variable resistance 
medium. 

Among the earlier forms of the granular transmitter is one de- 
signed by £mile Berliner, and called the "Berliner Universal.'' In 




FIG. 44.-THE BERLINER UNIVERSAL TRANSMITTER. 



this the diaphragm, D (Fig. 44), is of carbon, and is mounted hori- 
zontally in a case formed of the two pieces, A and B, of hard rubber. 
a brass ring, R, being clamped above it to insure good electrical 
contact. Secured to the enlarged head, f, of the screw, j, mounted 
on the block, B, is a cylindrical block of carbon, on the lower face 
of which are turned several concentric V-shaped grooves. The 
points formed between these grooves almost touch the diaphragm. 
The finely divided carbon, c, rests on the diaphragm, and is confined 
in the space between it and the carbon block by a felt ring, F, which 
surrounds the latter and bears lightly against the diaphragm. To 
the center of the back plate a soft rubber tube, r, is fixed which is 



60 



AMERICAN TELEPHONE PRACTICE. 



of sufficient length to make contact with the diaphragm, its function 
being that of a damper to the vibrations of the diaphragm. The 
mouthpiece, M, is so curved as to conduct the sound waves against 
the center of the diaphragm. This transmitter was once used to 
some extent by the American Bell Telephone Company, but has 
long since been entirely replaced for long distance work by the 
White transmitter. 




FIG. 45.— SECTIONAL VIEW OF SOLID-BACK TRANSMITTER. 



The White, or "solid back," transmitter, as it is called, is shown 
in Figs. 45 and 46, the latter giving a clear idea of the construction 
of the working parts. Fig. 45 shows the section of the complete 
instrument. The sections of the bridge piece, P, shown in Figs. 45 
and 46 are taken on planes at right angles to each other. This in- 
strument has proven remarkably successful in practice, it being able 



CARBON TRANSMITTER. 



61 



to stand a very heavy current without undue heating. Besides 
this, the tendency of the granules to settle down in a compact mass, 
commonly called "packing," is greatly diminished. It is without 
question one of the most successful transmitters yet introduced, and 
for a long time was unapproached in general efficiency. 

The front, F, is of brass, and is held, as shown, in the hollow shell, 
C, the two pieces forming a complete metallic casing for the working 
parts of the instrument. The sound-receiving diaphragm, D, of 
aluminum, is encased in a soft-rubber ring, e, held in place by two 



if t 




FIG. 46.— DETAILS OF SOLID-BACK TRANSMITTER. 



damping springs, / /, as in the Blake transmitter. IV is a heavy 
metallic block hollowed out, as shown, to form a casing for the 
electrodes. The inner circumferential walls of this block are lined 
with a strip of paper, i. This block is mounted, as shown, on a 
supporting bridge, P, secured at its ends to the front casting, F. 
The back electrode, B, of carbon is secured to the face of the metal- 
lic piece, a, which is screw-threaded into the block, /['. E is the 
front electrode, also of carbon, carried on the face of the metallic 
piece, b. On the enlarged screw-threaded portion, p, of the piece, 
b, is slipped a mica washer, m, held in place by the nut, u. This 
washer is of sufficient diameter to completely cover the cavity in the 
block, IV, when the electrode is in place. After the required amount 



62 AMERICAN TELEPHONE PRACTICE. 

of granular carbon has been put into the cavity, and the front elec- 
trode put in position, the cap, c, is screwed in its place on the block, 
W, as shown, and binds the mica washer, in, firmly against the face 
of the block, B, thus confining the granules in their place. The elec- 
trodes are of somewhat less diameter than the paper-lined interior of 
the block, IV, so that there is a considerable space around the 
periphery of the former, which is filled with carbon granules. This 
prevents the binding of the free electrode against the edge of its 
containing chamber, and also allows room for the granules directly 
between the electrodes to expand when heated by the passage of 
current. The screw-threaded portion, p' , of the piece, b, passes 
through a hole in the center of the diaphragm, and is clamped firmly 
in place by the nuts, t t'. M is the mouthpiece of hard rubber, screw- 
threaded in an opening in the front block F. Any vibration of the 
diaphragm is transmitted directly to the front electrode, E, which 
is allowed to vibrate by the elasticity of the mica washer, m. The 
back electrode is, of course, stationary, being firmly held by the 
bridge, P. 

The back electrode is in metallic connection with the frame of 
the instrument, which forms one terminal. The other terminal is 
mounted on an insulating block, /, and is connected by a flexible 
wire with the front electrode, E. 

This transmitter is now used on all of the long-distance lines of 
the Bell Companies, and has given excellent service. 

The following data concerning the dimensions and material used 
in this instrument will, it is believed, be found of interest : 

Diaphragm — aluminium, 2\" diameter and .022" thick. 

Rubber band gasket — 5" wide, 2§" double length, very soft and 
elastic. 

Front electrode — carbon, hard and polished, 21-32" diameter, 
1 -16" thick. 

Back electrode — carbon, hard and polished, 11-16" diameter, 
1 -16" thick. 

Mica diaphragm — 27-32" diameter, very thin. 

Back electrode chamber — inside diameter, J ", depth 5-32", clear- 
ance between sides of electrode and walls of chamber 1-32". 

Distance between electrodes about .04". 

Damping spring — spring steel, n-32" wide, .010" thick, 1 7-16" 
long; bent at right angles when not in place. The one which rests 
near center of diaphragm is tipped with soft rubber and also with 
felt ; the outer spring, with rubber only. 



CARBON TRANSMITTER. 



63 



In Fig. 47 is shown a sectional view of the standard transmitter of 
the Kellogg Switchboard & Supply Company, and in Fig. 48 a de- 
tail of the working parts of this transmitter. This instrument is one 
of the most carefully designed and constructed, and gives excellent 
results in practice. The front, A, is of cast metal, similar to that 
used in the solid back transmitter shown in Figs. 45 and 46, and, as 
in the case of that transmitter, all of the working parts are mounted 
upon it. The diaphragm, B, is of aluminum, and differs from that 
of any other transmitter now on the market, in that instead of being 




FIG. 47.— THE KELLOGG TRANSMITTER. 

a plane it has formed up in its center part a cup, b, which contains 
the electrodes and the variable resistance medium. The formation 
of this cup from a piece of thin sheet aluminum, which, by the re- 
quirements of its service, must be hard rather than soft, has proved, 
as the writer knows by experience, a difficult problem in metal draw- 
ing, and is interesting on that account, if on no other. The front 
electrode, C, is composed of a brass screw, r, having an enlarged 
head to which is soldered a hard carbon disc, c' . This is secured by 
means of a nut to the front inner face of a chamber formed in the 
diaphragm in a manner readily understood from the assembled 



64 



AMERICAN TELEPHONE PRACTICE. 



drawing. The rear electrode, D, is similarly made up of a carbon 
disc, d', soldered to a brass disc, d, having a prolonged screw- 
threaded shank. The shank of the rear electrode, D, passes through 
a mica washer, £, and a heavy brass bushing, F, having an enlarged 
face to clamp the mica washer against the rear face of the electrode. 
These parts are tightly clamped together by means of a small nut, f. 
After assembling the parts thus far, the proper amount of granular 
carbon is poured into the chamber, b, on top of the front electrode, 
after which the rear electrode, with the mica washer, is put in place, 
and the mica washer is firmly riveted to the diaphragm by means of 




FIG. 48.— DETAILS OF KELLOGG TRANSPUTER. 



an aluminum ring, G, and the small rivets, g. Under the heads of 
one of the rings is fastened a small clip, g', by which electrical con- 
nection is afterwards made with one of the terminals of the trans- 
mitter. The diaphragm of the transmitter is thus made, to carry 
with it the chamber and both electrodes, the chamber being perma- 
nently closed by means of the mica washer riveted in place, as de- 
scribed. The diaphragm thus assembled is surrounded by a soft 
rubber ring, b', held in place against the front piece by two heavy 
damping springs, as in the case of the solid back transmitter. The 
bridge, H, is secured at its ends to the rear face of the front, A. This 



CARBON TRANSMITTER. 



65 



bridge carries lit its center the heavy bushing, h, into which the bush- 
ing, F, of the rear electrode fits. The screw, h\ serves to hold the 
rear electrode rigidly with respect to the bridge after it has once 
been adjusted. 

A terminal block, /, of hard rubber, is secured to the rear side 
of the bridge upon which is mounted a terminal, i, to which a wire 
leading from the clip, g', is soldered. The lug, i, therefore forms 
one terminal of the transmitter, it being in electrical contact with 
the front electrode. The other terminal is formed by the frame of 
the transmitter itself, it being in contact with the rear electrode 
through the bridge, H. 




FIG. 49.— THE STROMBERG-CARLSON TRANSMITTER. 



In this transmitter the piston action of the electrodes is obtained 
as in other forms, the rear electrode being held rigid while the front 
electrode vibrates with the diaphragm. The flexible mica washer 
serves to allow a slight relative motion between the two electrodes, 
this washer also serving to completely close the chamber to prevent 
the escape of the granules or the entrance of moisture. Besides 
the piston action of the electrodes, it will be seen that practically the 
entire mass of granules is caused to be agitated by the vibration of 
the diaphragm, and therefore a certain amount of microphonic 
action may be expected from this cause alone. Certain it is, how- 
ever, that this is an exceedingly powerful form of transmitter. It 



66 



AMERICAN TELEPHONE PRACTICE. 



was developed by Mr. W. W. Dean after a long and most careful 
series of experiments. 

In Figs. 49 and 50 is shown the transmitter of the Stromberg- 
Carlson Telephone Manufacturing Company, Fig. 49 showing a 
sectional view of the transmitter assembled, and Fig. 50 the various 
working parts in detail. The working parts are all contained 
within a two-piece cup, the two parts of which are permanently 
riveted together, as shown at a a, after the transmitter is assembled. 
By thus permanently closing the cup it is not easy to tamper with 
the working parts, which as a result are usually left in their original 
state. The diaphragm, D, of this transmitter is of metal, but in 
front of it is placed an auxiliary diaphragm, D' , of silk, the two 
diaphragms being separated from each other by a ring, d, resting 



L 



E J & F 



E 




FIG. 50.— DETAILS OF STROMBERG-CARLSON TRANSMITTER. 



between them. Against the rear of the metal diaphragm rest two 
double-armed damping springs, these being secured to the rear of 
the case and serving to hold the diaphragm against the front of the 
case and also to prevent too great an amplitude of its vibration. 
Riveted to the center of the diaphragm is the front electrode, E, this 
consisting of a circular piece of woven wire gauze, heavily gold 
plated. The rear electrode, £', is in the form of gold-plated wire 
gauze also, pressed into cup shape and adapted to fit tightly over 
the enlarged head of the screw-threaded brass stud, F. A wire 
ring, G, is placed within the cup, E, before it is placed on the head, 
F, thus serving to prevent the inner face of the wire-gauze cup from 
coming into actual contact with the face of the head of the stud, F. 
A flanged collar, H, is forced tightly over the gauze cup, E, after it 



CARBON TRANSMITTER. 



67 



lias been placed on the head of the stud,, and this serves to clamp the 
gauze tightly on the stud. 

Cemented to the front face of the flanged collar, H, is a thick 
washer of very light plush, /, this virtually forming the cylindrical 
wall of the chamber containing the granular carbon. The granular 
carbon used is fine enough to pass through the meshes of the back 
electrode, E, and this occupies the space between the rear face of 
this electrode and the front face of the stud, F, this space being 
caused by the presence of the ring, G. between the two. The space 
in front of the electrode, E', and between it and the front electrode, 
E, is almost entirely filled with granular carbon, as shown in the 
assembled drawing. The screw-threaded portion of the stud, F, 
which carries the rear electrode, engages an internal screw-thread 
in the heavy rubber bushing, /, mounted within the rearwardly 
projecting collar extending from the cup, and by means of turning 





FIG. 51.-THE COLVIN TRANSMITTER. 

this stud in this bushing the adjustment of the transmitter electrodes 
may be effected. A machine screw, K, passing through a washer 
as shown, serves to engage a tapped hole in the end of the stud, F, 
and thus bind the latter in place in the rubber bushing, /. 

This transmitter has been put into use by customers of the 
Stromberg-Carlson Company to a very large extent and it has given 
uniformly good service, having proved to be thoroughly reliable, 
and remarkably free from the troubles which often beset granular 
carbon transmitters. 

In Fig. 51 is shown a transmitter designed by Mr. F. R. Colvin 
which is unique in its mode of action. This was at one time put 
into quite extensive commercial use by Mr. Colvin, but it is not 
now used. 



68 



AMERICAN TELEPHONE PRACTICE. 



The shell is formed of two pieces, A and B, of wood, the former 
carrying the mouthpiece. The space in which the diaphragm fits 
is made large enough to hold the diaphragm very loosely so that it 
may vibrate with great freedom. Upon the diaphragm, which is 
of aluminum, is supported a hollow cylindrical cell, D, of insulating 
material (shown in the small cut at the left), carrying two metallic 
electrodes, E E' , insulated from each other. To these electrodes are 
connected the circuit terminals, G G. The shell, D, is clamped 
firmly to the diaphragm, C, by a bolt, F, thus closing the chamber 
containing the granules. To prevent the access of moisture to the 
cell, the joint between the diaphragm and its edge is hermetically 
sealed by an adhesive compound. The striking feature of this in- 
strument is that the two electrodes, E E', are fixed with relation to 
each other, the variation in resistance being obtained by the varia- 




FIG. 52.— THE SUTTON TRANSMITTER. 



tion in pressure between the electrodes and the carbon granules, 
due to the inertia of the latter, and also to the shaking up of the 
granules themselves, and the consequent variation of their intimacy 
of contact with each other. 

Fig. 52 shows a transmitter typical of a large number of instru- 
ments made by the various independent manufacturing companies. 
This particular one was called the Sutton transmitter. 

The variable resistance parts comprise a pair of carbon buttons, 
F and G, each surrounded by a sleeve of cloth, H and /, the abutting 
edges, h and i, of which are frayed out so as to form an intimate but 
yielding contact. These form with the buttons, F and G, a closed 
chamber in which the granular carbon is placed. The button, F, 
is secured to the diaphragm, K, as shown, while the button, G, is 
rigidly secured to the case of the instrument, and is insulated there- 
from. The wire, O, leading from the bolt, L. which secures the but- 



CARBON TRANSMITTER. 



69 



ton, G, in place, forms one terminal of the instrument, the casing 
itself the other. 

The Ericsson transmitter, manufactured in Sweden, is being im- 
ported into this country to a considerable extent as a companion 
piece to the Ericsson receiver. This transmitter gives a very clear, 
soft tone, and requires little battery power. On the whole it is a 
very efficient instrument, except where great power is required. It 
is shown in section in Fig. 53, in which a is the sound-receiving dia- 
phragm held against a shoulder in the brass casing, c, by two thin 
leaf-springs, not shown, each spring having two branches, so as 
to give in all four points bearing on the diaphragm. 

For preventing moisture, especially that of the breath, from enter- 
ing beyond the diaphragm a thin disc, b, of silk impregnated with 
lacquer is placed in front of the diaphragm. 

The metal plate, d, mounted on the rear side of the diaphragm 




FIG. 53.— THE ERICSSON TRANSMITTER. 



forms the front electrode, and for that purpose is gold-plated. 
The backwardly bent rim of the plate, d, surrounds the forepart 
of a soft ring, e, on the carbon block, /, and serves to prevent the 
carbon grains from falling out of the chamber. This soft ring is 
made of raveled felt, and therefore allows the free vibration of the 
diaphragm. 

The diaphragm is damped by the coiled spring in a chamber in 
the center of the carbon electrode. This spring rests on a tuft of 
cotton or felt, which in turn bears on the center of the front elec- 
trode. 

The transmitter of the old Western Telephone Construction Com- 
pany (Fig. 54) is probably the simplest ever manufactured. The 
whole front case, A, of the transmitter is of a turned brass casting. 
Tt is shouldered inside to fonn a seat for the diaphragm. D, and 



70 AMERICAN TELEPHONE PRACTICE. 

threaded to engage an insulating cup, B, carrying the back electrode, 
C. This cup is screwed directly on a flange of the supporting arm, 
E, from the inside. The central screw which holds the back elec- 
trode in place also passes into the arm, thereby making the arm one 
terminal of the transmitter. The back electrode, C, is large, being 
I \" in diameter and §" thick. The chamber in which this block 
is mounted allows about \" space all around the electrode, which 
space, as well as that between the diaphragm and the back electrode, 
contains granular carbon. The diaphragm, D, is of carbon, usually 
.016" thick and 2 3-16" diameter, the free portion being 1 13-16" in 
diameter. The distance between the back electrode and the dia- 
phragm is 5-64". The chamber is only half filled with granular car- 




FIG. 54.— WESTERN TELEPHONE CONSTRUCTION COMPANY'S 
TRANSMITTER. 



bon, and only the lower half of the diaphragm, therefore, is actively 
engaged as an electrode. 

In Fig. 55 is shown a transmitter patented by Mr. T. F. Ahearn, 
changes in area of contact without changes of pressure. 

£ is a carbon electrode attached to the center of the metal dia- 
phragm, A, and forms the terminal electrode to which the w T ire, D, 
is attached. This electrode consists of a plate or plates, of either 
semi-circular or triangular form, as shown. 

The back electrode, G, is of similar form and is carried on the 
spring, /, in such manner as to overlap and rest on the front elec- 
trode, E. The pressure between the two may be regulated by the 
thumb-screw, as shown. 

It is claimed by the inventor that in this no variation in pressure 
can be caused by the vibration of the diaphragm, but that the elec- 



CARBON TRANSMITTER. 71 

trodes simply slide over each other, the shape of the surfaces in con- 
tact amplifying the changes in contact area. 

In granular-carbon transmitters much trouble has been experi- 
enced with what is commonly known as ''packing." This consists 
in the granules assuming such relation among themselves as to form 
a more or less compact mass, thus preventing the diaphragm from 
vibrating properly, and also* failing to act as a variable resistance 
medium to the vibrations of which the diaphragm is capable. This 
may be due to a variety of causes. Sometimes, where the granules 
are of varying sizes, and particularly where there is a considerable 
amount of fine dust mingled with them, they tend to arrange them- 
selves in layers in accordance with their size, the small ones working 
toward the bottom. In this state the entire mass of granules may 
become very compact, particularly at the bottom of the chamber, the 





FIG. 55.-THE AHEARN TRANSMITTER. 

fine dust tending to almost entirely fill the interstices between the 
larger particles. 

Probably a more common cause for packing is that a few of the 
granules may become wedged between the front and back elec- 
trodes, thus preventing the vibration of the front electrode. Some- 
times, in response to a very heavy sound wave, while the diaphragm 
is at the limit of its return stroke and the electrodes therefore at the 
farthest possible distance apart, the mass of granules will settle 
momentarily between the electrodes, and the diaphragm will be 
unable to spring back to its normal position. This may be demon- 
strated in almost any granular-carbon transmitter by placing the 
lips firmly against the mouthpiece, and drawing in the breath so as 
to draw the diaphragm forward. Upon releasing the pressure the 
instrument will probably be found to be perfectly dead. This is a 



72 AMERICAN TELEPHONE PRACTICE. 

trick often resorted to by salesmen to throw a bad light on a com- 
petitor's transmitter. To remove the possibility of packing from 
this cause to a large extent, most manufacturers are now either 
drilling a hole through the side of the mouthpieces close to the base 
or are making slots in the mouthpiece near its base so as to prevent 
the formation of a partial vacuum in front of the diaphragm. 
Almost any transmitter when packed may be again put in working 
condition by a sharp rap from beneath, as with the fist. Some 
transmitters, notably those where the chamber is mounted on the 
diaphragm, as in the type shown in Fig. 47, will gradually unpack, 
due to the action of the sound waves in speaking. This is due to 
the fact that the whole chamber vibrates with the diaphragm, thus 
tending to more effectually shake up the granules than when the 
front electrode alone vibrates. 

In order to prevent packing various forms of carbon-granules 
have been used, among others those of spherical shape. None of 
these, however, have proved so effective in practice as the irregu- 
lar shapes caused by merely crushing the carbons. The greatest 
pains should be taken, however, to secure granules of a uniform 
size, only those being accepted which will pass through a sieve 
having a certain sized mesh and which will not pass through a 
sieve having a slightly smaller mesh. 

Before transmitters had reached their present state of perfection 
in design and construction, many attempts were made to insure 
against packing by means of mechanical agitators, some of which 
worked automatically. A type which was widely used employed 
means whereby all of the working parts of the transmitter, includ- 
ing the outer case, could be rotated by merely twisting the mouth- 
piece. The form of transmitter shown in Fig. 54 was at one time 
arranged in this manner, the result being that if, at any time, it 
appeared dead this condition could be relieved by turning it over, 
allowing the granules to rearrange themselves. Another scheme 
for accomplishing the same result automatically was to provide 
mechanism for revolving the transmitter, a little at a time, at every 
stroke of the switch hook. A ratchet was mounted on the periphery 
of the transmitter casing, which was engaged by a pawl carried by 
the switch hook. All of these devices proved futile and have been 
abandoned. They were based on wrong principles, their object 
being to correct an evil rather than to prevent it. Moreover, they 
introduced unnecessary complexity which the whole tendency of 
telephone progress is to eliminate. 



CHAPTER VI. 
INDUCTION COILS FOR LOCAL BATTERY TELEPHONES. 

It has already been pointed out in Chapter II. that the use of the 
induction coil in connection with the local battery and variable 
resistance transmitter, is advantageous in that it allows the changes 
in the resistance of the transmitter to bear a much larger ratio to 
the totai resistance of the circuit in which these changes occur than 
would be the case were the same transmitter and battery placed 
directly in series in a line of comparatively high resistance; and 
further, that by virtue of the transformation from a comparatively 
low to a high voltage, the currents are much better adapted to 
traversing long lines and higher resistances. It may be further 
pointed out that with the same battery power the current in the 
primary circuit is much greater, owing to the lower resistance, than 
if the same battery were placed in the line circuit, and therefore the 
transmitter is not only able to produce a greater relative change in 
the current flowing, but to cause these changes to»act on a larger 
current. 

Such a system of transmission as uses a battery, associated with 
an induction coil at each telephone, is termed ''local battery" trans- 
mission, and such a telephone a "local battery" telephone. Recent 
tendency in a large class of telephone work has been to dispense 
with the local battery, using instead a single large battery for sup- 
plying a large number of lines and instruments from a single central 
point. Such systems of transmission are termed "common battery" 
or "central energy" transmission, and this distinction must be kept 
in mind. The present chapter, as its name indicates, deals only 
with induction coils for local battery work. 

It should be remembered that in local battery transmission the 
current in the primary circuit is an undulating one, being always in 
the same direction. The current in the secondary, however, is 
alternating in character, its direction depending on whether the 
primary current is increasing or diminishing in strength. 

The quality and dimensions of the iron core, the relation between 
the number of turns in the primary and secondary windings, and 
the mechanical construction of the induction coil are matters of 



AMER1CAX TELEPHOXE PRACTICE. 



importance, and have not in general received the attention thev 
merit. A number of attempts have been made to calculate mathe- 
matically the best dimensions and resistances of the telephone induc- 
tion coil, but the matter is of such an extremely complex nature, 
and all of the quantities are subject to such complex and almost 
indeterminate variations, that the results so far produced have been 
in general uncertain. 

Fig. 56 shows a sectional view, and Fig. ?j a view in perspective 
of a coil which in its general method of construction is tvpical of 
practice to-day. This coil, which is that formerly used bv the 
Western Telephone Construction Company with the transmitter 




FIG. 56.-SECTIONAL VIEW OF INDUCTION COIL. 




FIG. 57.— PERSPECTIVE VIEW OF INDUCTION COIL. 

shown in Fig. 54, has. however, a few eccentricities which may prove 
instructive. The core, C, is formed of a bundle of about 500 strands 
of Xo. 24 B. & S. gauge Swedish iron wire, and is 4 inches in length 
and 9-16 of an inch in diameter. The spool is formed of a thin 
fiber tube, T , over the ends of which are slipped the heads. E, of 
similar material, the parts being glued together. On this core are 
wound about 200 turns of Xo. 20 single silk-covered wire. This is 
two layers deep, so that the ends of the primary both emerge from 
the same end of the coil. Over the primary winding are wrapped 
several layers of oiled paper, after which the secondary is wound, 
this consisting of about 1400 double turns of No. 34 wire, two in 
parallel. These two wires are wound side by side throughout their 



INDUCTION COILS. 75 

length, and give the equivalent area of one No. 31 wire. The re- 
sistance of the primary coil is .38 ohm and that of the secondary 
75 ohms. The terminals of the secondary coil are shown at a b and 
a b in Fig. 56. After the coil is wound, the small wires of the 
secondary are attached to larger wires inside of the spool-head, so 
that the danger of breakage will be diminished. These leading-out 
wires are coiled in a tight spiral, as shown in Fig. 57, in order to 
avoid breakage and also to give a considerable length of wire in 
making connections where it is needed. 

More recent experience shows that a smaller diameter of core 
than that used in the above-described coil gives better results, the 
smaller core losing, perhaps, a little in loudness, but gaining per- 
ceptibly in clearness and crispness. The use of two fine wires in 
parallel, as found in this coil, is illustrative of a worn-out fallacy. 




FIG. 58.— INDUCTION COIL WITH TERMINALS ON HEADS. 

This practice was once quite commonly resorted to in various coil 
windings for telephone use, on account of some fancied theoretical 
gain in efficiency. The gain, however, was not real and the practice 
was undesirable, expensive and useless. Undesirable, because two 
small wires are more easily injured mechanically than one large wire 
of the same carrying capacity ; expensive, because the labor in wind- 
ing is considerably greater than in the case of a single wire, as is 
also the first cost of the finer wire. 

In modern coils a paper tube is generally used in place of the fiber. 
as, being thinner, it allows the winding to be placed closer to the 
core, an advantage in point of efficiency and economy. 

A much better method of terminating the wires leading from an 
induction coil than that of ending them in "pigtails" is to solder 
them directly to brass or German silver contacts secured to the coil 



76 



AMERICAN TELEPHONE PRACTICE. 



heads as shown in Fig. 58, or, where the coil is mounted on a sepa- 
rate base, to similar clips mounted on the base as shown in Fig. 59. 

By either of these methods the liability of breaking the wires of 
the coil close to the head, or within the head, rendering repair 
difficult or impossible, is largely obviated. A coil built in this way 




FIG. 59.-INDUCTION COIL WITH TERMINALS ON BASE. 

is much more readily connected in the circuit of a telephone, on ac- 
count of the substantial nature of its terminals. 

A coil constructed as shown in Fig. 60 is being manufactured 
by the Varley Duplex Magnet Company. The core consists of a 
bundle of small cables, each composed of seven strands of rather 
fine Swedish-iron wire. On this the primary, consisting of three 





FIG. 



-THE VARLEY INDUCTION COIL. 



layers of cotton-covered magnet wire, is wound. The secondary 
is wound in two sections, the right-hand head of the spool being 
made removable, so that the sections may be removed for mak- 
ing repairs. Bare wire is used in winding the secondary, the 
adjacent convolutions of the wire being held apart by a fine thread 



INDUCTION COILS. 



77 



of silk wound alongside and parallel with the wire, as shown in 
Fig. 61. This method of winding coils is old, having been invented 
by Dr. Leverett Bradley and patented by him in 1865 \ but it has 
recently been introduced by the Varley Company in the various 
branches of telephone work with much success. A layer of paper 
is introduced between each layer of wire, and in this way the insula- 
tion is made complete. The machines for winding in this manner 
have been perfected with such nicety that several coils are simul- 
taneously wound, the layers of paper being automatically introduced 
between each layer of winding without stopping the machinery, 
which is run at a very high speed. Considerably more wire can be 
placed on a coil in a given space than with the ordinary method of 
winding; and the fact that bare wire is used tends to render the 
coil cheaper. 

This same company has carried the idea of sectional windings 
throughout the entire field of telephone work, They construct their 




THREAD 
FIG. 61.-MANNER OF WINDING VARLEY COIL. 



spools in such manner that the heads may be readily removed 
and a coil replaced' without the necessity of rewinding. 

It is now quite common to mount the induction coil in the base 
of the arm on which the transmitter itself is mounted, such con- 
struction being shown in Figs. 62 and 63. The base and arm here 
shown are made of cast iron joined in such manner as to allow a 
considerable vertical movement of the transmitter, in order to ac- 
commodate it to the heights of different users. The coil has some- 
times been mounted upon the back board of the telephone, so as to 
be covered by the base of the arm when secured in place, but a more 
desirable method is to mount it in the arm-base, as shown, the vari- 
ous terminals being brought out to binding posts on the front of the 
base. This construction, however, is bad, unless well carried out, 
and great pains should be taken in insulating the various posts and 
wires from the conducting base. A considerable advantage has been 
claimed for this type of coil mounting, due to the presence of the 



78 



AMERICAN TELEPHONE PRACTICE. 



iron case about the coil, thus rendering the magnetic circuit more 
complete. This, however, is a point of doubtful validity, as it may 
be claimed with equal force that the presence of the case gives rise to 
undue impedance and to eddy currents which would have a detri- 
mental effect. As a matter of fact, the presence of the case has little 




FIG. 62.— TRANSMITTER MOUNTED ON ARM. 

appreciable effect one way or another on the quality of the trans- 
mission. 

Only a few series of experiments are on record from reliable 
sources giving the results of comparative tests between induction 
coils of various dimensions. It may be said that definite results from 
any such series of tests are hard to get, as the quality and loudness 




FIG. 63.— INDUCTION COIL IN BASE OF ARM. 



of transmission is subject to more or less personal error, even in the 
case of experienced experimenters. 

A series of experiments, cited by Preece and Sttibbs and per- 
formed by the adminstration of the Swiss telephone department, 
is of interest. In this test a o-ood Blake transmitter was used 



INDUCTION COILS. 



79 



throughout, the object being to determine the best of a set of ten in- 
duction coils. Table I. gives the most important data concerning 
the primary and secondary windings of each coil. 

TABLE I. 





Ppimary Winding. 


Secondary 
Winding. 


Results for Various Lengths of Line. 


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38 miles 


49 miles 


53 miles. 


67 miles 


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1956 


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3191 


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180 


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62 


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4080 


35 


250 


.9 


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1.3 


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1.3 


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1 


4.. 


116 


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3952 


35 


250 


1.5 


1.3 


1.7 


15 


1.3 


1.5 


1 3 


1.5 


1.2 


15 


5.. 


230 


24 


1.00 


3865 


35 


250 


1.3 


1.0 


1.3 


1.2 


1.1 


1.3 


1.3 


1.5 


1.0 


1.3 


6.. 


Xii 


24 


1.20 


4420 


35 


300 


1.5 


.9 


1.6 


.0 


1.7 


13 


1 7 


1.6 


1.5 


1.5 


?'.. 


295 


24 


1.50 


4278 


35 


300 


1.3 


.9 


1.5 


.9 


1 1 


1.1 


1.5 


1.4 


1.6 


1.3 


8.. 


368 


24 


2.00 


4735 


35 


350 


1,3 


1.0 


1.5 


.9 


11 


1.0 


1.5 


1.4 


1.6 


1.2 


9.. 


368 


21 


1.17 


4735 


29 


130 


1.7 


10 


1.6 


.9 


1.7 


1.4 


1.6 


1.6 


17 


1.3 


10. 


1350 


24 


10.00 


3950 


35 


400 


.3 


.3 


.8 


.5 


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.8 


.3 


.4 


.3 


.1 



The results obtained overdrive different lengths of line are shown 
in the right-hand portion of the table. In each case the intensity 
and clearnes of the Blake transmitter with a standard coil was 
taken as unity, and the results are expressed in terms of this stan- 
dard. The resistance of the primary wire of this standard coil was 
1.05 ohm and that of the secondary 180 ohms. It will be noticed 
from the results that coils Nos. 4, 6 and 9 were, all things considered, 
the best, while coils Nos. 1 and 10 were very inferior. The table 
also shows in general that a coil that was good for a short distance 
was also good for a long distance, and this is perhaps the most in- 
sti uctive lesson to be gained from these tests. It is hard to draw 
any definite conclusions from the performances of the various coils 
as to their relative merits and to point out why coils Nos. 4, 6 and 9 
should give better results than the others, or why coils Nos. 1 and 
10 should be so much inferior. It shows, moreover, that good re- 
sults may be obtained with the same transmitter and with coils dif- 
fering widely as to their characteristics ; this being shown particu- 
larly in the case of coils Nos. 4 and 9, the former having a second- 
ary of 250 ohms and a primary of -| ohm, while the latter had a sec- 
ondary of 130 and a primary of 1.17 ohm. The coil adopted for the 
Blake transmitter in this country has a primary winding of 4 ohm 
and a secondary of 250 ohms, which, it will be seen, corresponds 
exactly to coil No. 4 in this table, which g-ave the best results. 



80 AMERICAN TELEPHONE PRACTICE. 

With modern transmitters for local battery work, which use a 
much stronger current than the Blake tendency among the manufac- 
turing concerns whose practice may be considered the best, is to re- 
duce the ratio of transformation by making the number of turns 
on the secondary windings very much lower than was formerly the 
case. As an extreme example of this, it may be cited that the coil 
recently used to a large extent with the solid-back transmitter on 
the long-distance lines of the American Telephone and Telegraph 
Company had a primary of .3 ohm and a secondary of but 14 ohms 
resistance. This coil was provided with a very large core composed 
of a bundle of soft-iron wires, and its total length between the heads 
was six inches. This' coil proved effective, but undoubtedly had 
too much iron in its core for clear articulation. Its use has therefore 
been generally abandoned. 

Table II. shows the principal data for the induction coils used for 
local battery telephones by several different manufacturers. These 
various manufacturers have evidently adopted the coil design, which 
seemed to them to give the best results with their particular trans- 
mitters. This table is of interest in showing how widely diversified 
the practice is in this regard. 

The matter of comparative tests for induction coils deserves some 
attention. Since the induction coil in the local battery telephone 
plays no part save in the transmission of speech, it follows that the 
best coil is that which will give the best transmission, and by best 
transmission is not meant, necessarily, that which is loudest, but 
that which, all things considered, is best and most easily understood 
over those lines on which the induction coil is to be used. 

It may be said in general that in making comparative tests as to 
the transmission efficiency of any piece of telephone apparatus, it 
is of great importance that all possibility of prejudice on the part 
of the experimenter be removed, and, in order to do this, it is de- 
sirable that he be in ignorance at all times of the particular instru- 
ment that he is testing. 

Another thing to guard against in making tests of this kind, is 
that the hearer shall not give undue consideration to the factor of 
loudness. That piece of apparatus which gives the loudest trans- 
mission is not necessarily the best, and even greater stress should 
be placed on the quality of tone and clearness. 

Tests are frequently made by reading certain subject matter to 
the listener, using one piece of apparatus in the transmission, and 
then reading the same matter, using another piece of apparatus for 



INDUCTION COILS. 



81 






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82 AMERICAN TELEPHONE PRACTICE. 

comparison. This is always unfair to the merits of the first piece 
of apparatus, for the listener will be more familiar with the matter 
on the second reading than on the first; will understand certain 
words the second time that he missed the first time, and is very 
liable to attribute this to better transmission rather than to the fact 
that he has had two chances instead of one of understanding the 
matter read. The method of reading passages of considerable 
length, first over one instrument and then over another, is also 
faulty, because the person transmitting cannot maintain the same 
loudness and distinctness of articulation throughout the entire test, 
nor can the person listening make a distinct comparison between 
what he hears over one instrument and that which he hears over 
another, on account of the lapse of time between the two tests. A 
better way of making transmission tests, and one which has been 
adopted by those most experienced in this line of work, is to so 
arrange the apparatus that the several instruments or apparatuses 
under test may be switched into and out of the circuit of the line 
instantly, so that no loss of time occurs between the tests. Instead 
of transmitting long sentences of unfamiliar matter, the person at 
the transmitting end repeats alternately over each instrument, some 
short and perfectly familiar matter, designating in each case the 
number of the instrument over which he is speaking. Good sub- 
ject matter for transmission in this way is by merely counting up 
to five on each instrument, and stating at the end the number of the 
instrument over which the count was made, thus: "i, 2, 3, 4, 5, 
on No. 1," and "1, 2, 3, 4, 5, on No. 2," etc. Constant repetition 
of this, first on one instrument and then on another, may be made 
without the transmitting party changing his tone, and the party 
listening may finally decide whether No. 1 or No. 2 is the better, 
by noting the several characteristics of each transmission and care- 
fully fixing them in his mind at each recurring time until he finally 
reaches a definite conclusion. He may then transmit his conclusion 
back to the sending party, informing him that No. 1 or No. 2 has 
made the better test. 

These are apparently unimportant points, but the quality of voice 
transmission on telephone lines is such a subtle characteristic that 
it is only by eliminating, as far as possible, the personal equation 
of the experimenters, that results of any value may be obtained. 

Coming more particularly to the subject of testing induction coils, 
Fig. 64 shows the circuit for making a comparative test between 
three different coils, using a fourth as a standard. The same set 



INDUCTION COILS. 



S3 



of apparatus is used at each end of the line, the circuit arrangement 
being such that by placing any one of the four-lever, double-throw 
switches to the left or right, the particular coil to which the switch 
thrown corresponds will be switched into or out of the proper 
circuit relation with the line, and the receiver, transmitter and bat- 
tery at each end. Throwing any switch will connect both primary 
and secondary coils simultaneously. When all of the switches are 
in their normal positions, the coil connected at the end of the series 
of switches, as that marked ''Standard" in the figure, will be con- 
nected. Thus, in making comparison between the "Standard" and 
any of the other coils, it is only necessary to throw the switch be- 




4»?P 





3TAMPAr\P STANPAKP 

FIG. 64.— CIRCUIT FOR COMPARATIVE TESTS OF INDUCTION COILS. 



longing to that coil which it is desired to compare with the 
"Standard." If coil No. 3 is to be used as a standard its switch 
would be thrown, and then coils No. 1 or No. 2 could be compared 
with it by throwing their respective switches. 

Mr. R. H. Manson, of Chicago, using circuits and apparatus 
arranged as shown in this figure, conducted a comparative test on 
a line extending between Columbus and Cleveland, Ohio. All of 
the coils, the data of which are shown in Table II., were used in 
this test, the Western Electric or Bell No. 16 coil being used as a 
standard. 

The conclusions arrived at in this experiment favored the coils 



84 AMERICAN TELEPHONE PRACTICE. 

having the small core and small winding area. It was found that 
the coils having large cores and large winding areas were uniformly 
louder but poorer in quality than those having smaller cores and 
smaller winding areas. 

Experiments were made in talking over a large coil at the trans- 
mitting station to a small coil at the receiving station and vice versa, 
each of these latter tests proving that the small coil would lose in 
transmitting but would gain in receiving. The small coil at the 
receiving end with a large coil at the transmitting end would always 
give better results than where two large coils were used together, 
and two small coils always gave better results, all things considered, 
than a large and a small coil. 

In tests using the same kind of coil at each end, it was found that 
on noisy lines the large coils, with their large volume of transmis- 
sion, were not as efficient as the small coils with their clearer but 
weaker transmission. 



CHAPTER VII. 
PRIMARY BATTERIES. 

If a sheet of zinc and one of carbon be separated from each other 
and immersed in a liquid capable of chemically attacking the zinc, 
a difference of potential will at once be formed between the two 
plates. If the two plates are then connected together by a wire, a 
current of electricity will flow from one to the other through the 
wire, and while the current is so flowing the zinc will be eaten away 
by the solution with more or less rapidity. Such a combination 
is called a voltaic cell, and two or more of such cells may form an 
electric battery. Of course other substances than zinc and carbon 
may be used, it only being necessary that both plates be of con- 
ducting material and that one of them shall be of such a nature as 
to be chemically attacked by the fluid. The two plates of the cell 
are called electrodes, and the solution in which they are immersed 
the electrolyte. 

The current is assumed to flow from the plate which is attacked 
through the electrolyte to the one which is not, and therefore in the 
cell under consideration from the zinc to the carbon plate. The 
plate which is attacked is therefore always called the positive plate 
or electrode, and the one which is not attacked the negative. 

Starting from the surface of the zinc, where the chemical action 
is taking place, the current flows through the electrolyte to the sur- 
face of the carbon electrode, thence by means of the wire back to 
the zinc electrode. 

It will be noticed that the current flows from the carbon to the 
zinc in the wire, outside the electrolyte; and therefore in order to 
make the terms positive and negative properly refer to the poles 
with respect to the current flowing in the external circuit, the 
carbon terminal is called the positive pole and the zinc terminal the 
negative pole. It seems at first a little confusing to have a positive 
pole on a negative plate, and a negative pole on a positive plate; 
but if the direction of the current be kept in mind as being always 
from positive to negative, no confusion will arise. 

The part of the circuit outside of the battery connecting the two 
poles is called the external circuit. The internal circuit is of course 

85 



86 AMERICAN TELEPHOXE PRACTICE. 

through the two electrodes and the electrolyte, and the resistance 
of this latter path is called the internal resistance of the cell. 

Zinc forms the active or positive electrode of most primary cells, 
while the negative electrode is usually of carbon or of copper. No 
matter, however, of what materials the electrodes are formed, that 
which is attacked by the electrolyte while the battery is in action 
forms the positive plate of the cell, the current flowing always from 
it in the electrolyte. 

In nearly all cases hydrogen is liberated from the electrolyte at 
the negative plate — that is, at the plate which is not attacked. This 
forms a film over the surfaces of the negative electrode which, un- 
less removed or destroyed, tends to greatly weaken the strength of 
the cell, for two reasons : first, the film of gas is of very high resist- 
ance, and therefore raises the internal resistance of the battery 
enormously, thus causing a correspondingly small flow of current; 
and second, the gas is itself attacked by the electrolyte, hydrogen 
having almost as great an affinity for the oxygen in the latter as has 
the electrolyte itself for the zinc. This causes a counter-electromo- 
tive force to be set up which to a large extent neutralizes that set up 
by the action of the electrolyte with the zinc. The phenomenon of 
the collection of hydrogen on the negative electrode in a cell is called 
polarization; and it is necessary to adopt some means to prevent 
it to as great an extent as possible, as otherwise a cell w 7 ould become 
useless after a very short period of use. 

The LeClanche cell, which is used to great extent for telephone 
work, has a negative electrode consisting of carbon and peroxide 
of manganese, a positive electrode of zinc, and an electrolyte of a 
solution of sal ammoniac. The sal ammoniac attacks the zinc, 
forming zinc chloride and liberating hydrogen and also ammonia 
gas on the surface of the carbon. The peroxide of manganese, 
which is usually in small lumps, closely associated with the carbon, 
is exceedingly rich in oxygen, which slowly unites with the free 
hydrogen to form water, thus getting rid to a large extent of the 
polarizing effect of the hydrogen. The peroxide of manganese is, 
however, not merely a depolarizer, as it is usually considered, but 
is an essential part of the negative electrode. This is proved by the 
E. M. F., which is as great as we have a right to expect between 
zinc and peroxide of manganese in a sal ammoniac solution, but is 
greater than we are justified in expecting or can obtain from a zinc 
carbon couple in a like solution. In use, cells of this type polarize 
rather quickly, but as soon as the external circuit is opened they 



PRIMARY BATTERIES. 



87 



slowly recover, owing to a combination of the hydrogen with the 
oxygen. This cell is therefore suitable only for cases where the 
circuit will be closed for a few minutes at a time; and this is exactly 
the condition met in telephony. 

The cell shown in Fig. 65 was once widely used by the Bell 
companies, one cell with each Blake transmitter. The zinc elec- 
trode is in the form of a rod, while the carbon electrode is imbedded 
in a porous pat of clay which is immersed with the zinc in the 
electrolyte. Around the carbon within the porous pot is packed a 
mixture of black oxide of manganese and broken carbon, the latter 




FIG. 65.— THE LECLANCHE CELL. 



to give greater conductivity to the mixture and a greater surface to 
the carbon electrode. 

A cell using practically the same materials for its various parts, 
but dispensing with the porous cup of clay, is shown in Figs. 66 
and 67, the latter being a sectional view. The carbon electrode is 
in the form of a corrugated hollow cylinder, 1 (Fig. 67). which 
engages by means of an internal screw-thread a corresponding 
thread on the under side of a carbon cover, 2. Within this cylinder 
is a mixture, 10, of broken carbon and black oxide of manganese. 

The zinc electrode, 6, is in the form of a hollow cylinder almost 
surrounding the carbon electrode, and separated therefrom by 



88 



AMERICAN TELEPHOXE PRACTICE. 



means of heavy rubber bands stretched around the carbon. The 
rod forming the terminal of the zinc passes through a porcelain 
bushing on the cover-plate, so that a short-circuit cannot take place. 
The terminal pin, 8, is imbedded as shown in a hole, 4, in the 
carbon cover, by first heating the cover to a high degree and then 
pouring in melted lead. This forms, with the nut, 7, and the washer, 
6, a very secure form of connector for the positive pole. Unless 
some such precaution as this is taken, corrosion soon sets in around 
the metallic connection to the carbon, thus causing a poor con- 
nection. These cells are used to a large extent by the independent 
telephone companies in this country. They have an electromotive 
force of about 1.55 volts, and recuperate very quickly after severe 
use. 

Another form of sal-ammoniac batterv differing: from that shown 




FIG. 66.-CARBON CYLINDER LECLAXCHE CELL. 



in Fig. 67 principally in that the parts of the negative element are 
contained in a canvas bag rather than in a porous cup of clay is 
shown in Fig. 68. This bag is suspended from the fiber plate form- 
ing the cover for the containing jar, which cover also has a hole 
through which the zinc rod forming the positive element may pass. 
This type has proved a success in practice and has largely replaced 
the type using the clay cup in those places where wet batteries are 
still used. 

Many other forms of sal-ammoniac cells are in common use. 
Some of these consist merely of a zinc rod hanging in the center 
of a carbon cylinder, no depolarizer being furnished. In other 
forms the carbons have molded with them the manganese depo- 



PRIMARY BATTERIES. 



89 



larizer and are in various shapes, but all act in the same general 
way. 

The advantages of the LeClanche type of cell for telephone work 
are many. They are inexpensive in first cost and in renewals. They 
are very cleanly, giving out no noxious fumes and containing no 
highly corrosive chemicals. They require almost no attention, the 
addition of a little water now and then to replace the loss due to 




FIG. 67.— SECTIONAL VIEW OF CARBON CYLINDER LECLANCHE CELL. 

evaporation being about all that is generally required. They give 
a rather high electromotive force and have a moderately low inter- 
nal resistance, so that they are capable of giving a considerable 
amount of current for a short time, and lastly, if properly made, they 
recuperate quickly after polarization due to heavy use. 

The following directions should be observed in setting up and 
maintaining LeClanche cells: To set up, place not more than four 
ounces of prime white sal ammoniac in the jar. Fill the jar one- 



90 



AMERICAN TELEPHONE PRACTICE. 



third full of water and stir until the sal ammoniac is all dissolved. 
Then place the carbon and zinc elements in place. A little water 
poured in the vent-hole of the porous-pot forms will tend to hasten 
the action. Unless a cell is subject to very severe use, it will require 
but little attention if it is a good one. Water should be added to 
supply loss by evaporation. If the cell fails to work, examine its 
terminals for poor connections. If the zinc is badly eaten, replace 
it with a new one. If this fails to improve it, throw out the solution 
and refill as at first. If now the cell does not work properly, the 
porous pot or carbon element may be soaked in warm water, and if 
this gives no better results they should be replaced. In the cells 
shown in Figs. 66 and 67, the depolarizer may be removed by un-' 
screwing the carbon from the cover. In commercial work, however, 




FIG. 68.— BAG TYPE LECLANCHE CELL. 

it seldom pays to attempt to repair a negative element after it has 
become ineffective. 

The Bell Companies before their almost universal adoption of 
common battery systems used in their long-distance work, in con- 
nection with the solid-back transmitter, another form of cell known 
as the "Standard" Fuller. In this the positive electrode is a heavy 
block of zinc molded into conical form around a heavy copper wire, 
which forms the negative pole. The negative electrode is a block 
of carbon hanging through a slot in a wooden cover. The separate 
parts are shown in Fig. 69. The zinc rests in the bottom of a 
porous cup when in place. The electrolyte for this cell is made as 
follows : 

Sodium bichromate 6 ounces 

Sulphuric acid 17 ounces 

Soft water 56 ounces 



PRIMARY BATTERIES. 



91 



Dissolve first the sodium bichromate in the water and then add 
slowly the sulphuric acid. (Never pour the water into the acid.) 
The mixture should be made in an earthen vessel, or if in a glass 
jar, the jar should be placed in cold water in order to prevent over- 
heating. 

Another solution called electropoin fluid may be used as the elec- 
trolyte in this cell. It is made of bichromate of potash instead of 
bichromate of sodium. 

The cell is set up according to the following directions: 

Place the quantity of solution made by the above formula in the 
glass jar. 

Put one teaspoonful of mercury in the bottom of the porous cup, 




FIG. 69.-PARTS OF "STANDARD" FULLER CELL. 



add two teaspoonfuls of common salt, place the zinc in the bottom 
of the cup, and fill to within two inches of the top with soft water. 

Place the porous cup in the jar and put on the cover, passing the 
wire from the zinc through the hole provided for it. The cell is 
then ready for use. 

The active element in the electrolyte in this cell is the sulphuric 
acid, which of course attacks the zinc. The bichromate of sodium 
or of potash serves as a depolarizer, the oxygen in it combining with 
the hydrogen, liberated at the positive pole, to form water. 



92 AMERICAN TELEPHONE PRACTICE. 

The character of the electrolyte, containing as it does a most 
vigorous acid, makes necessary great care that the proper methods 
and materials be used in the construction of Fuller cells. 

The following specifications governing the furnishing of these 
cells have been used with good results: 

One cell of "Standard" Fuller battery shall consist of the follow- 
ing parts: I glass jar; I wooden cover; i carbon plate with binding 
post and lock-nuts; I cast zinc; i porous pot — all as hereinafter 
specified. 

Glass Jar: The glass jar shall be of first quality flint glass, 
cylindrical in form, 6 inches in diameter and 8 inches in depth. 

Wooden Cover: The cover shall be of clear, kiln-dried white- 
wood. It shall be thoroughly coated with two coats of asphalt 
paint, and be of such dimensions as to form a proper cover for the 
jar. 

Carbon Plate: The carbon plate shall be of rectangular form 
and approximately 4 inches wide, 8| inches long, and J inch thick, 
no dimension to vary more than 1-16-inch. It shall be of good 
quality, homogeneous and free from flaws, cracks, and other de- 
fects, and completely carbonized. Each carbon shall be provided 
with a suitable clamp or terminal. The parts of the clamp shall be 
of bronze, and shall be nickel-plated. Before attaching the clamp 
to the carbon, the carbon shall be heated in a temperature of at least 
250 degrees Fahrenheit, and the top portion of it shall be immersed 
in paraffin at a temperature of about 250 degrees Fahrenheit, the 
immersion to continue until the immersed portion of the carbon is 
saturated. After the clamp is attached to the carbon, but before the 
lock-nuts are in place, the carbon shall be immersed in melted 
paraffin at a temperature less than 170 degrees Fahrenheit. The 
carbon plate is then to be completed by attaching the lock-nuts. 

Cast Zinc: The zinc shall be in the form of a truncated cone 2\ 
inches in diameter at the base, 2.\ inches high and 1 inch in diameter 
at the top. It is to be made of Rich Hill spelter. Cast into the 
zinc shall be a soft copper wire .1018 of an inch in diameter (No. 10 
B. & S. gauge) . The wire is to extend 8 inches above the zinc. The 
zinc and the copper wire shall be amalgamated to a height of 4 inches. 

Porous Pot: The porous pot shall be cylindrical in form, 3 
inches in diameter and 7 inches deep. 

The Fuller cell made according to the above specifications gives 
an E. M. F. of 2.1 volts, and was found by the New York Tele- 
phone Company to be the most practical cell then available for its 



PRIMARY BATTERIES. 



93 



heaviest telephone service. A still more powerful cell, and one some- 
what more convenient to handle, is shown in Fig. 70. 

In this the zinc is very heavy, and in order to present a greater 
surface to the electrolyte has a horizontal cross-section in the form 
of a cross. The carbon electrode is in the form of a hollow cylinder 
completely inclosing the porous pot. The carbon cylinder has a 
flaring top provided with a flange which fits over the upper edge 
of the glass jar, thus forming a very complete cover for the entire 
cell. 

The following are the data given by a manufacturer concerning 
the main points of this form of Fuller cell: 

E. M. F v 2.1 volts. 

Current, about 8 amperes. 






FIG. 70.-PARTS OF FULLER CELL WITH CYLINDRICAL CARBON. 



Carbon, 4^ inches diameter by 8^ inches over all. 

Carbon surface exposed to solution, 156 square inches. 

Zinc weighs 2 pounds; 2\ inches across; total length, 8 inches. 

Zinc surface exposed, 54 square inches. 

Porous cup, 3 inches diameter, 7 inches long. 

Jar, 6 inches diameter, 8 inches deep. 

Solutions same as "Standard" Fuller cell. 

Cell, complete, weighs 8 pounds 12 ounces. 

The internal resistance of Fuller cells is very low, especially in 
the cylindrical carbon type. They will stand for several months 
on open circuit with but little local action. 

Formerly three cells in series, giving six volts, were used in local 



94 



AMERICAN TELEPHONE PRACTICE. 



battery work with the solid-back transmitter, but it has been found 
that two cells give, all things considered, as good or better results. 

Still another form of battery, of entirely different type, is shown 
in Fig. 71. This is known as the gravity battery, and is used to a 
very large extent in telegraph service, and also in telephone work 
where it is necessary to have a small but constant current always 
flowing. In this cell the negative electrode is of sheet copper, 3 
strips of which are riveted together at their centers, after which the 
ends are bent outwardly, so as to present a large surface to the 
electrolyte. The zinc is in the form of a "crow foot," cast with a 
lug adapted to hook over the edge of a glass jar. In setting up 
this battery the copper is first put in place in the bottom of the jar. 
Sulphate of copper, or blue vitriol, as it is called, is then filled in 




FIG. 71.-THE GRAVITY CELL. 



around the copper to a height almost sufficient to cover it. The 
jar is then filled with water and the zinc put in place. 

In this battery sulphuric acid is formed, which attacks the zinc 
to produce zinc sulphate. This fluid is lighter in weight than the 
solution of copper sulphate and therefore occupies the upper por- 
tion of the cell. The fact that the two solutions in this battery are 
kept apart by gravity instead of by the use of a porous pot, as in 
the Fuller cell, is accountable for the name, "gravity cell." As the 
zinc sulphate is colorless, while the copper sulphate is of a dark- 
blue color, the separating line between the two liquids is easily 
distinguished. This line is termed the "blue line," and should be 



PRIMARY BATTERIES. 



95 



kept about midway between the copper and the zinc. If the blue 
line rises too high, so as to come in contact with the zinc, it should 
be lowered. This can be done by short-circuiting the battery for 
a short time, or by drawing off some of the blue fluid with a siphon 
and filling in with water or with zinc sulphate from another bat- 
tery. In cases, however, where the battery is in constant use, it 
very rarely happens that the blue line reaches too high a level, and 
the reverse is more likely to take place. If the blue line reaches 
the upper portion of the copper, more crystals of bluestone should 
be dropped in, and if this does not remedy the difficulty some of 
the zinc sulphate from the top of the cell should be siphoned out 
and replaced by clear water. These batteries are very satisfactory 




FIG. 72.— THE GORDON CELL. 



for closed-circuit work, but are not well adapted to telephone work 
in general on account of their high internal resistance. 

A cell of more recent origin than any of the types so far described 
has come into limited use for certain classes of telephone work. 
This is the Gordon cell, shown, assembled and in parts, in Fig. 72. 
The negative element consists of a perforated tin cylinder, filled with 
black oxide of copper, which is a powerful depolarizing agent. 
Three porcelain lugs or cleats are attached by iron bolts to the 
lower portion of the tin cylinder, and upon these rest the rolled zinc 
cylinder forming the positive element. This electrolyte is a strong 
solution of an alkali termed by the manufacturers ''electro-sodium." 
It is probably simply caustic soda. The size of the zinc for a 6" x 8" 
cell is 5-J" in diameter and 2.\" wide, containing about \\ pound 
of amalgamated zinc. A No. 12 B. & S. gauge insulated copper 



9G AMERICAN TELEPHONE PRACTICE. 

wire fastened to the zinc extends through a porcelain bushing in 
the cover and forms the negative pole. The positive pole is formed 
of a metal rod passing through a porcelain bushing in the center 
of the lid. This rod serves to support the entire negative element 
to which it is fastened. After the cell is set up complete a layer of 
oil is poured over the top of the solution to prevent evaporation. 

The electromotive force of this cell is low in comparison with 
most other cells, being very close to .66 volt under working con- 
ditions. The internal resistance is about .04 ohm. The 6" x 8" 
cell has a capacity of 300 ampere-hours when discharged at a rate 
varying from one to six amperes. Larger sizes of cells are made, 
with capacities said to be as high as 1000 ampere-hours. 

This cell is adapted to open or closed-circuit work, particularly 
the latter, where a small current is required steadily for a long time. 
It has the advantage of requiring little or no attention until com- 
pletely exhausted, of maintaining a constant electromotive force 
throughout its life, of being non-freezing at all ordinary tempera- 
tures, and of being free from local action. It is frequently used as 
a reserve battery in small common-battery exchanges to supply 
current, if for any reason the charging current for the storage 
battery fails. It is, however, not economical to use it for the regu- 
lar source of current supply in such exchanges, as has often been 
proven. 

When any battery is idle there should be no action between the 
electrolyte and the zinc. This would be the case were it economical 
to use perfectly pure zinc, but inasmuch as commercial zinc always 
contains impurities, frequently consisting of other metals, a local 
galvanic action is set up, the impurities forming with the zinc 
minute galvanic couples. In order to reduce this action to a mini- 
mum, it is advisable, especially in such cells as the Fuller, to 
amalgamate the zinc — that is, to coat it with mercury. This seems 
to give a perfectly homogeneous surface to the zinc, which prevents 
local action. The fact that this local action takes place on account 
of impurities in the zinc makes it very clear that the quality of metal 
used is a matter of very great importance. 

A type of primary cell, known as the dry cell, is coming into 
constantly increasing favor in telephone work. These cells are 
not strictly dry, for their very action depends on the presence of 
moisture. If they become really dry chemical action ceases, and 
with it their capabilities to generate a current. The term "dry cell" 
is not, however, amiss, as these cells have none of the disadvantages 



PRIMARY BATTERIES. 



'.u 



due to the possible spilling of liquid possessed by all of the cells 
so far described. 

In these cells the electrolyte, instead of being in the form of a 
free liquid, is held absorbed by some porous substance, such as 
sawdust, blotting paper, or like material, the cell being sealed to 
prevent evaporation. 

One of the best of these dry cells is shown in section in Fig. 73. 





FIG. 73.— DRY CELL. 



The outer casing is of zinc, carefully formed into a cup, so that it 
serves not only as a retaining chamber but also as to the positive 
electrode. The negative electrode is a carbon rod. shown in detail 
at the right. This is held in the center of the cup. out of contact 
with it, the intervening space being filled with a mixture of peroxide 
of manganese, powdered carbon, and some moisture-retaining 
porous substance, the whole being saturated with a solution oi sal 



98 AMERICAN TELEPHONE PRACTICE. 

ammoniac. The exact formulas used by various manufacturers are 
not made public. A cylinder of several thicknesses of blotting- 
paper lining the inside walls of the zinc chamber serves to prevent 
the carbon and manganese from coming directly in contact with 
the zinc. 

After assembling, the zinc chamber is closed with a substance 
resembling sealing wax poured in while hot, and an outer casing of 
pasteboard put on. This outer casing has no other function than 
to insulate the zinc cup from surrounding objects. The end of the 
carbon plate projecting through the sealing material is provided 
with a binding post, serving as the positive pole of the cell, while 
an upwardly extending rod from the zinc casing carries a binding 
post which serves as the negative pole. 

The early forms of dry cells were uniformly condemned by tele- 
phone men, and justly so, as they proved to be wholly unreliable. 
This fact, together with the fact that some of the old forms are still 
manufactured, has served to prevent the adoption of dry cells in 
some localities, to as great an extent as their real merit warrants. 

It is now believed that for local battery telephone work the dry 
cell, when properly made, is superior to any of the wet forms of 
zinc-carbon cells using sal ammoniac as an electrolyte. They have 
been so perfected chemically and electrically that they have as 
great, or greater, outputs, and better recuperative power than any 
of the other types of LeClanche batteries, while in point of con- 
venience and economy, their small size and non-breakable and non- 
spillable features and low cost leave no room for comparison. 

Among companies using large numbers of local battery tele- 
phones, the question of the electrical efficiency and of the life of the 
battery is of much importance, yet the choice of a battery is too 
often made on account of low first cost or on account of the claims 
of the selling agent rather than on the basis of a practical com- 
mercial test. The significance of this will be apparent when it is 
stated that among the well-known dry cells now on the market in 
the United States, there is a difference in point of "telephone life," 
in the ratio of about four to one. In other words, the best types 
will remain effective in actual telephone service practically four 
times as long as the poorer types. This would mean that in ex- 
changes where the poorer type of cells was used, the batteries would 
have to be renewed practically four times as often as would be 
necessary if the better grade were used under the same conditions. 
Thus, beside the actual cost of the cells, the company is forced to 



PRIMARY BATTERIES. 



99 



pay for labor and carfare for four renewals, where only one would 
be necessary with the better cell. 

Frequently battery tests are made by simply short-circuiting the 
cell for a given period, or by closing its circuit permanently through 
a certain resistance and noting the time taken for a cell to com- 
pletely discharge or to discharge until its voltage is reduced to a 
certain amount. These methods are not fair and lead to erroneous 
conclusions. These conditions are not those of practical telephony; 




jKOHMS / ° " 

r t\ \ TWO &-L-J \—^ 

' Uuf Iceil sJC 




FIG. 74.— METHOD OF TESTING PRIMARY BATTERIES. 

cells designed for open-circuit work only being thus tested under 
the conditions of work that would be given to a closed-circuit cell. 

In Fig. 74 is shown in diagram a method of making comparative 
battery tests which may be made to closely approximate the actual 
conditions of service. A clock is equipped with three contacts on 
its face, each adapted to be engaged once each hour by a wiping 
contact carried by the minute hand. Each of these contacts extends 
over an arc equal to that traversed by the minute hand in the time 
during which it is desired to have the battery on closed circuit. 



LofC. 



100 



AMERICAN TELEPHONE PRACTICE. 



say five minutes. The circuit will thus be closed in the clock for 
a period of five minutes three times each hour, there being an inter- 
mediate open-circuit period of fifteen minutes between each closure. 
Any ordinary ''dollar" clock may easily be thus equipped. 

In circuit with the contacts of the clock are placed in series, the 
windings of a number of relays, and a battery of 24 volts, as shown. 
A relay is provided for each battery to be tested and each relay is 
adapted when operated to close the circuit of its battery through a 
resistance equal to the average resistance of the transmitter with 
which the battery is designed to be used. 

In order to facilitate the daily reading of the current and voltage, 
two double-contact spring jacks, a and v, are connected, as shown, 




44 48 52 56 GO 



FIG. 75.— CURVES OF BATTERY TESTS, CONSTANT-RESISTANCE METHOD. 



with the circuit of each battery under test. The jack a is so con- 
nected as to include between its terminals both the coil and the 
battery under test, thus giving the reading of the current when a 
plug in circuit with the milliammeter is inserted into it. The jack 
v is connected across the terminals of the battery only, so as to make 
a direct reading of the voltage when a similar plug connected with a 
voltmeter is inserted into it. 

The results of a set of tests made by Mr. R. H. Manson with 
apparatus thus arranged are shown plotted in the form of curves 
in Fig. 75. In these tests each battery was closed through its re- 
sistance for a period of six minutes three times an hour. All 



PRIMARY BATTERIES. 101 

readings of each battery were taken at intervals of twenty-four 
hours, while the relay contacts were not closed; in other words, 
while the batteries were on open circuit. Inasmuch as the current 
was found at all times to be substantially proportional with the 
voltage, the voltage readings alone were deemed a sufficient indi- 
cation as to the condition of the cell. These curves, therefore, show 
the voltage of each battery for every day during the test. In this 
test it was assumed that when a battery of two cells dropped in 
potential below one and a half volts, it was unfit for service. For 
this reason a horizontal line has been drawn across the curve sheet 
at a height indicating one and a half volts, and the number of days 
required by each battery, under the conditions of the test, to fall 
below this voltage, is shown by the point at which the curve crosses 
this horizontal line. Reference to the curves will make clear that 
battery A lasted sixty days before dropping to this voltage, while 
the battery C lasted not quite twelve days. Battery D was some- 
what better in this respect than battery C, although both showed 
poor results. Battery B was somewhat inferior to A, although 
both of these were first class.' 

If we assume as a basis of comparison that in a given exchange 
there are ten calls a day, lasting three minutes each, we have thirty 
minutes a day of actual use of the battery. As battery A was in 
use during the test in all 25,920 minutes, it would seem that under 
the assumed conditions of practice, using a local circuit having a 
resistance of 20 ohms, this battery would have a telephone life of 
864 days. Battery C would have a life of about 173 days. These 
figures are probably too high, although there are cases on record 
where dry batteries have given service for over two years without 
any attention whatever. 

One reason why the figures mentioned are probably too high, is 
that in practice where the service would ordinarily be very much 
less severe than that imposed during the test, the batteries would 
stand idle for a much longer time. During this time it is probable 
that there is a slight amount of chemical action and this would, of 
course, serve to reduce the actual life of the battery. 

In making tests of this nature it is an easy matter for the ex- 
perimenter to vary the conditions to meet the requirements of his 
exchange. The coil through which the discharge takes place 
should, of course, be made equal to the average combined resist- 
ance of the transmitter and primary winding of the induction coil. 
The clock is easily made to close the circuit more frequently, and 



102 



AMERICAN TELEPHONE PRACTICE. 



for a less period of time, if desired. Again, in order to more nearly 
approach the actual average conditions of telephone service, the 
circuit of the clock may be opened at night so as to give a complete 
rest to all the batteries during the time when telephones are least 
busy. Inasmuch, however, as it is usually desirable to obtain the 
results of such a test without waiting too long a period, the condi- 
tions of the test, cited are recommended as giving fair results. 

A modification of the method of battery testing just described 
has been used by some of the large Bell companies. This involves 
changing the resistance in the circuit with each cell under test from 
time to time so as to keep the current delivered by the batteries 
constant throughout the test. 

1.6 



fcx 
































1.2V 




--*..^ 




— ^^ 


























0\ 


N 




-— 1 ~ 


■5^7. 




> 


* — 
















.8 




\ 


\ 








^ — "-^ 






""*"««* 


"*^» 


-X°-^ 






-2 






\ 


\ 


















> 






A 






\ 


;t t 














■% 














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\ \ 
\ \ 

















Ho 


urs 






s. \ 


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\ 




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20 40 60 80 100 120 140 1G0 180 200 220 240 260 280 300 
Hours duration of test. 

FIG. 76.— CURVES OF BATTERY USING CONSTANT-CURRENT METHOD 



In Fig. 76 are shown the results of testing four different types of 
dry cells, these results being in each case the average of six cells 
of each type, four of the most prominent makes early in the year 
1902 being represented. The circuit was alternately closed and 
opened five minutes at a time, conditions being in this respect more 
severe than those in the tests the results of which are shown in 
Fig. 75. By changing the resistance in circuit with each cell the 
current was kept as nearly as possible to two amperes throughout 
the entire time of the test. These curves, therefore, show the 
ability of a cell to produce a current equivalent to two amperes. 
As soon as any cell was unable to deliver two amperes it was dis- 
carded. The solid lines show T the voltages taken on closed circuit, 



PRIMARY BATTERIES. 103 

and the dotted lines the voltages taken on open circuit, the readings 
being taken always at the beginning of the open-circuit periods. 
As a result of tests under these conditions we find that the poorest 
cell failed in a little less than 120 hours, while the best one lasted 
about 310 hours. The reason for keeping the current constant at 
two amperes is not apparent. This condition is not met in practice 
and it seems that the constant resistance method is to be preferred. 

Either of these sets of curves show several facts of importance. 
First, among standard makes of dry cells of good repute some are 
vastly superior to others. Second, that cell which gives the high- 
est voltage at the start and maintains it for a considerable period 
after the beginning of service is not necessarily the best; in fact, the 
reverse is apt to be true. 



CHAPTER VIII 

MAGNETO CALLING APPARATUS. 

So far we have dealt solely with the apparatus by which the 
actual transmission of speech is accomplished. While these are, 
of course, the most vital parts of a complete telephone, they would 
be of little use were not means provided whereby one party might 
call the attention of another in order to bring about a conversation. 

Ordinary vibrating bells, using current derived from a battery, 
were at first used for calling, and as the battery for operating the 
transmitters could also be used for this purpose, this plan seemed 
tc offer many advantages. It was found, however, that the voltage 
furnished by a telephone battery was insufficient to operate call-bells 
at great distances. 

Another way of using the energy of the battery for calling, is to 
use an induction coil to obtain a high voltage, impulses in alternate 
directions being sent to line by making and breaking the primary 
circuit in which the battery is placed. In this way practically as 
high voltage as is desired may be obtained, and bells adapted to 
respond to alternating currents may be rung at great distances. 
This method has been put into practice in several instances, but it 
is found to produce too heavy a drain on the battery and thus im- 
pair its life and effectiveness for talking purposes. 

The magneto generator is largely used for generating the calling 
current. Its use was almost universal in exchange work until the 
comparatively recent advent of the so-called common battery or 
central energy systems. In spite of the well-deserved popularity 
of these latter systems, the magneto generator still has and, it is 
thought, will have a very wide field of usefulness in telephony, on 
account of its reliability and adaptability to purposes and conditions 
for which common battery calling is not available. 

The magneto generator is the simplest form of the electric 
dynamo, and consists of an armature of the Siemens type, wound 
with many turns of fine wire, and so mounted as to enable it to be 
rapidly revolved between the poles of a permanent horse-shoe mag- 
net. Its theory of action is very simple and depends on the prin- 
ciples of magneto electricity discovered by Faraday and Henry, 

104 



MAGNETO CALLING APPARATUS. 



105 



and pointed out in a previous chapter — that if the number of lines 
of force passing through a closed coil be varied, currents of elec- 
tricity will be generated in this coil, the direction of these currents 
depending upon the direction of the lines of force and on whether 
their number is decreasing or increasing. 

In Fig. 77 is shown a simple loop of wire, a, which may be re- 
volved about a horizontal axis in the field of force of a permanent 
magnet. The horizontal arrows represent the direction of the lines 
of force set up by the magnet through the loop. Suppose the loop 
to be turned in the direction of the curved arrow. When it is in 
the horizontal position no lines of force will pass through it. As it 
approaches the position shown by the full line it will include a 




FIG. 77.— FIELD OF FORCE IN MAGNETO GENERATOR. 

larger and larger number of these lines. The current induced in 
the coil will then be in the direction indicated by the arrows, x, and 
will so continue until the loop is in its vertical position. The num- 
ber of lines passing through the loop then begins to decrease, and 
the current therefore takes the opposite direction, as indicated by 
the arrows, z. The current increases in strength in this new direc- 
tion until the coil is horizontal. At this point the rate at which the 
number of lines through the coil is changing is greatest, and the cur- 
rent is therefore a maximum. As the coil passes through the horizon- 
tal position the number of lines passing through it begins to increase 
again. This would cause another change in the direction of the 



106 



AMERICAN TELEPHONE PRACTICE. 



current, were it not for the fact that the direction of the lines of 
force through the coil also changes. The same cycle of events takes 
place during the next half-turn, when the coil is in the position from 
which it started. 

We thus see that the current generated is an alternating one, 
changing its direction twice during every revolution. 

In practice the armature, instead of having a single turn of wire, 
as in Fig. JJ, has a great number of turns of fine wire wound on a 
cast-iron core of the form shown in Fig. 78. In this figure, A 
represents a shuttle-shaped core of cast iron, on which the coils of 
wire, w, are wrapped. One end of the wire forming the coils is 
fastened to the pin, p, which is fastened to and is in metallic con- 
nection with the core, A. The other end is fastened to the pin p' , 
which is insulated from the core, but connects with the pin, c, pro- 
jecting from the end of the armature shaft and is insulated there- 




A G A- 

FIG. 78.— ARMATURE OF MAGNETO GENERATOR. 



from by the fiber bushing, b. Projections, a a, integral with the 
core, are turned down to form bearings for the armature. A 
pinion, f, is carried on the end of the shaft, in order to transmit to 
the armature the motion received from a large driving-gear wheel 
with which it meshes. 

A magneto generator in connection with a call-bell is shown 
diagrammatically in Fig. 79. To the poles of the permanent mag- 
nets, TV S, of the generator are attached cast-iron pole-pieces, P P, 
bored out so as to allow the armature, A, to turn freely between 
them. The bearings of the armature are usually mounted on brass 
plates firmly attached to the ends of the pole-pieces, but not shown 
in this figure. By means of a crank attached to a suitable gear 
wheel engaging a pinion on the armature shaft, the armature may 
be made to turn rapidly. 

As the currents generated are alternating, a polarized bell or 



MAGNETO CALLING APPARATUS. 



107 



ringer is needed. C C are the two coils of an electromagnet, pivoted 
in front of the poles of which is a soft-iron armature, A, carrying a 
hammer, H, on the end of a thin rod extending it at right angles 
from the center of the armature. A permanent magnet, N S, is so 
mounted as to magnetize by induction the armature, A, and the 
cores of the coils, C C. 




_ & _ 



<-> 



<-> 



PC 



n 



FIG. 79.-DIAGRAM.QF GENERATOR AND BELL. 

The two poles of the electromagnet will thus have a given polarity, 
say, north, while the two ends of the armature will have an opposite 
polarity, south. As a result, the armature will have a tendency to 




FIGS. 80 AND 81.-DETAILS OF TYPICAL MAGNETO GENERATOR. 

stick to one pole or the other of the magnets. The two coils are 
oppositely wound, and when a current passes through them it 
strengthens the magnetism of one pole and weakens that of the 
other. The next instant the current reverses, and the strong pole 



108 



AMERICAN TELEPHONE PRACTICE. 



becomes the weaker, and vice versa. As a result the armature 
vibrates with each reverse of current and causes the hammer, H, to 
strike the bells, B B. 

The details of a typical magneto generator are shown in Figs. 80 
and 81. This instrument is very similar to the one used by the 
Bell companies. In it the pole-pieces are of cast iron, riveted to- 
gether by means of the shouldered brass rods, B B. After this they 
are bored by a special tool to the required internal diameter to 
receive the armature. The core of the latter is of cast iron and is 
shown in Fig. 82, being accurately turned to fit freely between the 
pole-pieces. The bearing plates are of cast brass with a shoulder 
also turned to fit between the pole-pieces so as to be self-centering 
when secured in place. They are each fastened to the ends of the 
pole-pieces by four screws, as shown. The gears are cut from heavy 




FIG. 82.— ARMATURE CORE OF TYPICAL GENERATOR. 



cast brass, the large gear being mounted on a shaft journaled in the 
same bearing plates as the armature itself. 

The magnets are bent cold from f " x J" magnet steel, and are 
secured in place by clamping plates, C C, and screws, 5 S, the latter 
passing between the magnets and into the pole-pieces. The air 
gap in machines of this type may be reduced to about 1-100 of an 
inch without endangering the smooth running of the armature. 

A typical form of magneto bell or ringer is shown, minus its 
gongs, in Fig. 83. This form has been in wide use and well made 
gives excellent results. In this the two coils are wound on cores of 
round Norway iron, the heads of the spools consisting of fibre 
washers forced on the ends of the cores. The cores are fastened by 
means of screws to a soft iron yoke, A, which serves as a support 
for the working parts of the ringer, in mounting it in the telephone 
box. 

The armature, B, is pivoted between two trunnion screws carried 
on bent-up ears from the brass strip, C The strip, C, is supported 



MAGNETO CALLING APPARATUS. 



109 



at each end between two nuts carried on the brass bars, D D y these 
bars being riveted, or otherwise secured, to the yoke, A. The per- 
manent magnet, E, is secured to the center portion of the yoke, A, 
at one end, its other end bending around the armature support but 
not touching it. The adjustment of the ringer may be altered by 
moving the armature support lengthwise on the rods, D D, by means 
of the nuts on these rods between which it is fastened. By doing 
this the armature may be made to approach or recede from the pole- 
pieces, thus altering the strength of the magnetic pull on the arma- 
ture, and also the length of stroke of the tapper which is carried by 
the armature. 

The gongs of this type of ringer are mounted independently of 




FIG. 83.— TYPICAL MAGNETO BELL. 

the striking mechanism shown in the cut and on the opposite side 
of the supporting board from this mechanism, the tapper wire or 
rod extending through a hole in this board, to allow the tapper to 
play between the gongs. Means are provided in the gong supports 
for allowing the adjustment of the gongs toward or from the tapper 
to meet the various requirements of service. The supporting 
board usually forms the front cover, or some other portion of the 
telephone box, and this arrangement, therefore, brings the working 
parts of the bell within the box so as to be protected from dust and 
accidental derangement, while the gongs are on the outside where 
their sound may be more easily heard. 

In later forms of ringers, this independent mounting of the gongs 



110 AMERICAN TEI EPHONE PRACTICE. 

is done away with, the whole ringer being made self-contained, 
which is a feature finding more and more favor in all branches of 
telephone design. 

The details of design and construction of magneto calling ap- 
paratus have received much attention, and improvement has been 
effected during the last few years, owing undoubtedly to the strong 
competition between manufacturers supplying this apparatus. The 
essential features of a good generator are that it should have an 
electrical output as great as possible ; that it should be so strongly 
and durably constructed as to secure good wearing qualities and 
permanent adjustments ; that its magnets should be permanent so 
as not to lose their strength in the lapse of time, and that it should 
be as nearly noiseless and smooth in its operation as possible. 

In order to secure the greatest output of electrical energy, it is, 
of course, necessary that the permanent magnets should be as 
strong as possible. With a given set of magnets, however, and 
therefore with a given available magnetizing force, the design and 
construction of a generator may be so altered as to give widely 
different results in regard to output. With a given magnetizing 
force, the number of lines of force extending from one pole of the 
magnet to the other will depend on the material between the poles 
and also on the distance between them. Certain substances, of 
which iron is pre-eminent, if placed in a magnetic field of force, will 
have set up in them a vastly greater number of lines of force than 
would air, if subjected to the same magnetizing force. Such sub- 
stances in which a given magnetizing force will produce a high 
degree of magnetization, are said to possess a high degree of per- 
meability. A piece of soft rod iron placed between the poles of a 
permanent magnet would allow more lines of force to pass through 
it than would a piece of hard cast iron. The soft iron is therefore 
said to be more permeable of the two. 

One important point in the design of magneto generators as, in 
fact, in dynamo design in general, is to cause as great a number of 
lines of force as possible to pass through the core of the armature. 
Since in a magneto generator the magnetizing force when once 
determined, is, for all practical purposes, invariable, there are three 
factors which determine the number of lines that shall pass through 
the armature core. First, the quality, as effecting permeability, of 
the iron in the armature core and in the pole-pieces; second, the 
cross section of the iron in the armature core and in the pole-pieces 
taken in a plane at right angles to the direction of the lines of force; 



MAGNETO CALLING APPARATUS. Ill 

and third, the amount of air gap between the armature core and the 
pole-pieces. From this we may reason that to secure a maximum 
efficiency the iron of the pole-pieces and of the armature core should 
be of the greatest possible permeability and cross-section, while the 
air gap should be as small as possible. The tendency has been 
during the past few years to construct armature cores by building 
them up of thin layers of soft sheet iron, forming what is known in 
electrical design as a laminated core. This practice generally suc- 
ceeded the old practice of building the armature cores of cast iron, 
but quite recently several manufacturers have gone back to the 
cast armature core, paying particular attention, however, to 
securing a very soft grade of iron. The argument for this latter 
practice is that with later designs it is possible to secure a greater 
cross section in an armature core, thus allowing substantially the 
same number of lines of force to pass through as with the smaller 
cross section and better quality of iron in the older types. 

A cast armature, however, even though soft, is subject to another 
objection, in that eddy currents are generated in the core, which 
interfere to some extent with the efficiency of the machine. The 
laminated core tends, of course, to greatly reduce the liability to 
eddy currents. This objection is not thought to be of much mo- 
ment, and it may be said in conclusion that excellent generators 
are now being manufactured, some of which use laminated cores 
and others cast cores. 

The length of the air gap between the armature core and the pole- 
piece is a matter to be determined by mechanical conditions. The 
gap should be made as small as possible without endangering the 
smooth running of the armature. If the air gap is made too small 
and the generator poorly built, a little wear or change of adjust- 
ment will cause the armature core to strike the pole-pieces at every 
revolution, thus destroying the running qualities of the machine. 
In very good generators, where sufficient care is taken in their 
manufacture to secure accurate dimensions and permanent adjust- 
ment, the air gap is sometimes reduced without practical difficulty 
to about i/ioo of an inch. Where generators are poorly made 
this distance should be correspondingly greater with a resulting 
loss of efficiency. 

The wire used in the armature winding should be of the largest 
size that will give the desired number of turns without unduly filling 
up the winding space. The winding space should not be sc com- 
pletly filled as to cause the wires to bulge out and strike the pole- 



112 



AMERICAN TELEPHONE PRACTICE. 



pieces of the generator in its rotation, thus wearing away the 
insulation and frequently breaking the wire itself. 

In a given generator rotated at a given speed the voltage will 
depend on the number of turns of wire in the armature. The best 
size of wire to use is therefore readily experimentally obtained by 
first finding the number of turns required to give the proper voltage 
on an open circuit, and then using the largest size of wire for which 
there is room to wind on that many turns. 

Cast iron has been almost universally used in the construction of 
the pole-pieces, although some generators have been made using 
soft sheet iron pole-pieces stamped and formed into the desired 
shape. This forms a cheaper pole-piece than can be procured by 
the use of cast iron, because the latter must necessarily be subjected 
to a considerable amount of machine work, such, for instance, as 





FIG. S4.— DETAILS OF GENERATOR POLE-PIECES. 



the boring of the concave cylindrical surfaces between which the 
armature revolves. A point in favor of the cast iron pole-piece is 
that the air gap may be made much smaller because of the greater 
accuracy that can be secured by boring than by forming the pole- 
pieces in dies. Another advantage of the cast pole-piece is that 
with them a greater cross section of iron may be secured in the 
path of the lines of force, as well as a more direct path, than by using 
any of the formed sheet iron pole-pieces so far produced. The 
only argument in favor of the sheet iron pole-piece, aside from its 
cheapness, is that the quality of iron is better, but this, in view of 
the great cross section available in the cast iron, and of the smaller 
air gap, is not of considerable importance. 

In the construction shown in Fig. 84 the pole-pieces are of cast 
iron firmly secured together by shouldered brass rods. After being 



MAGNETO CALLING APPARATUS. 113 

thus fastened together they are bored out with a special tool, after 
which the magnets are put in place and clamped by any suitable 
means. This is a very good, although somewhat expensive, con- 
struction when properly done. 

The efficiency depends to a considerable extent upon the form 
of the current wave generated by the machine. This is governed 
largely by the relation between the width of the pole of the armature' 
and the distance between the flat surface of the generator pole- 
pieces. In Fig. 84 the best relation between these dimensions is 
illustrated quite clearly. It will be noticed in the figure at the left 
that the curved portion of the armature pole exactly corresponds 
to the concave portion of the pole-pieces, while in the figure at the 
right, which shows the armature in a different position, the poles 
of the armature are just sufficient in width to bridge across the 
space between the pole-pieces without overlapping. 1 

The sine wave has been found to be most efficient in the ringing 
of magneto bells, especially on lines of considerable length and 
possessing a high degree of self-induction and capacity; and the 
relation between the armature poles and the pole-pieces shown in 
the above figure gives a fair approximation to this form of wave. 
Where the armature poles do not fill the space between the pole- 
pieces, the current curve will have four distinct humps in each 
complete cycle. There will be a break in the magnetic circuit just 
as the armature pole leaves the pole-piece on one side, which will 
cause a sharp fluctuation in the electromotive force; and another 
sharp fluctuation will occur immediately after, when the opposite 
points of the armature poles approach the corners of the pole- 
pieces. These two fluctuations will occur twice in each cycle. 
When the armature poles are so wide as to overlap, when in the 
position shown in the right-hand portion of Fig. 84, the wave is 
flattened unduly and does not, therefore, give as high an electro- 
motive force as could otherwise be obtained. 

The effective pressure of the ordinary magneto generator, when 
rotated at the ordinary speed by hand, is from 65 to 75 volts, and it 
may be made, of course, higher or lower to meet certain require- 
ments by winding with a greater or less number of turns or by 
gearing the armature so as to rotate with greater or less speed. 

Some telephone lines, as for instance party lines, using a large 
number of instruments in series, require magneto generators 
capable of producing a very high electromotive force in order to 
successfully overcome the great resistance offered. Inasmuch as 



114 AMERICAN TELEPHONE PRACTICE. 

all of the bells are in series, the current required is not large. The 
ordinary generator for series work has a three-bar field, and has its 
armature wound to a resistance of about 550 ohms with No. 35 B. 
& S. gauge wire silk insulated. 

In a bridged line, however, where all of the ringer magnets are 
connected across the line in parallel, the current required is heavy, 
while the voltage need not, as a rule, be so high. In long lines of 
this latter type using a high-resistance wire, it becomes necessary 
to develop enough pressure to overcome the resistance of the line 
wire in order to ring the bell at the farthest end, and also a sufficient 
current to pass in multiple through all of the ringers. In this case 
a rather high voltage is required and a heavy current, so that the 
total amount of energy is large. This greater amount of energy 
cannot be obtained by merely winding the armature to a higher 
resistance, and therefore in generators of this type it is customary 
to use heavy and very powerful permanent magnets and to exercise 
the greatest care in the construction to produce the highest effi- 
ciency. Instead of using three permanent magnets, as in the series 
type of generator, it is customary to use four or five, the armature 
and pole-pieces being made correspondingly longer. A good gen- 
erator for bridging work may be wound with No. 33 B. & S. gauge 
wire to a resistance of 350 ohms. 

The construction of the polarized call-bell, or ringer, is a matter 
requiring no less attention to detail than that of producing an 
efficient generator. The old form of ringers, using a cast iron 
frame polarized by small permanent magnets, was subject to very 
grave defects. The frame became readily polarized in one direction 
or the other, due to the passage of a heavy current through the 
magnets, and would thus give the armature a set to one side or the 
other, which frequently succeeding currents of a weaker nature 
could not overcome. This, with the fact that with every reversal 
of the current the entire magnetic field set up through this heavy 
mass of poor quality iron had to be completely reversed, was a point 
rendering the construction of an efficient ringer almost an impos- 
sibility. The tendency in the present form of ringers is to make a 
magnetic circuit which is subjected to the changes due to the mag- 
netizing force as short as possible and of the very best material. 
Swedish or Norway iron, cold drawn and annealed, has been found 
to meet these requirements most perfectly. The sticking of the 
armature to one pole or the other is further prevented by the inter- 
position of a thin sheet of non-magnetic material, usually copper, 



MAGNETO CALLING APPARATUS. 115 

between the faces of the armature and the pole-pieces. Sometimes 
this is accomplished by inserting a small rivet either into the center 
of the pole-piece or into the armature face itself. 

The length of the rod carrying the hammer plays a considerable 
part in the sensitiveness of the bell. A long rod will secure for the 
hammer a long and powerful stroke, but the sensitiveness is cor- 
respondingly reduced. On the other hand, a short rod will produce 
a short and comparatively weak stroke, but the bell will be more 
sensitive than with the long rod. 

The ordinary construction of ringer magnets is to drive the fiber 
heads directly onto the cores, and after insulating the surface of the 
latter to wind the spools thus formed with silk-insulated wire of the 
desired size. Fig. 85 shows one of the sectional coils of the Varley 
Duplex Magnet Company, this being wound separately, with bare 
Avire separated by silk thread. After winding, the coils may be 
slipped on the core and locked by the end washer or head. The 




FIG, S5.-VARLEY RINGER COIL. 

advantages of this construction are in the ease of replacing burned- 
out or otherwise injured coils, and also in the perfect uniformity of 
the winding. 

The ringer magnets for series work are usually wound with No. 
31 B. & S. gauge silk-insulated wire and have a resistance of from 
75 to 100 ohms. 

Ringer magnets for bridging work must possess a very high 
degree of self-induction. This is obtained by winding them to a 
high resistance with a comparatively coarse wire, so as to obtain a 
large number of turns. The length of the cores is increased for the 
double purpose of getting more iron in the magnetic circuit, and 
therefore a higher retardation, and also for affording a greater 
amount of room for the winding. The Western Electric Company 
wind their bridging coils to a resistance of 1000 ohms, using No, 33 
single-silk magnet wire. Some companies use No. 38 wire and 
wind to a resistance of 1200 or 1600 ohms. This does not Efive 



116 AMERICAN TELEPHONE PRACTICE. 

such good results, however, as using the coarser wire and the lower 
resistance with long cores. Some companies wind, or once wound, 
their bridging-bell magnets partly with German silver wire, in order 
to make a high resistance at a low cost. Resistance in itself, how- 
ever, is not the thing desired, but a great number of turns in the 
winding, which, of course, incidentally produces a high resistance. 

The standard series generator and ringer for ordinary exchange 
work are usually so wound that the generator will ring its own 
bell, or another like it, through a resistance of 10,000 ohms. Such 
an outfit is spoken of as a 10,000-ohm magneto, and the 10,000 
refers not to the resistance of the bell magnets or the generator 
armature, as is often supposed, but to the external resistance 
through which they will successfully work. 

A good bridging generator should ring twenty 1000-ohm bells in 
multiple over a resistance of 1000 ohms between the generator and 
the first bell. 

On account of the high resistance of the generator armature and 
its great retarding effects, it is, under some circumstances, desirable 




FIG. 86.— MANUALLY OPERATED SHUNT. 

to have the armature winding shunted out of the line when the 
generator is not in use. Especially is this desirable where more 
than two telephone instruments are placed in series in the same 
circuit. Under these conditions, unless some by-path is afforded 
around the armature of the generators that are not in use, the voice 
currents and also the ringing currents from other stations would be 
compelled to traverse the winding of the generator armatures not 
in use, thereby reducing both the talking and ringing efficiency. 
Such devices are usually called "generators shunts." These are of 
two types, automatic and manual. The manual shunt is indicated 
in principle in Fig. 86. In this, a and e represent the two wires 
leading from the generator, the circuit through the generator 
between these wires being traced from a to spring b, which bears 
against the terminal pin of the armature, thence from the wire, c, 



MAGNETO CALLING APPARATUS. 



117 



forming the winding, to the frame, d, of the generator, and to the 
wire, c. »Its path, however, is normally short-circuited by means 
of the manually-operated spring, k, normally resting against the 
back contact, g, in such manner as to complete a circuit of prac- 
tically no resistance between the terminals, a and e. Under such 
circumstances, any currents coming over the line would follow the 
shunt path rather than traverse the armature. Pressure on the 
button, h, however, will break the shunt path, under which circum- 
stance a current generated in the armature winding, upon operating 
the generator, would pass to line, not being able to traverse the 
shunt path. 

Automatic devices have now almost entirely supplanted the man- 
ual, as the latter were never satisfactory, owing to the inability or 
ignorance of careless persons to properly manipulate them. Many 




FIG. 87.— AUTOMATIC SHUNT. 



styles of these automatic shunt devices have come into general use, 
the one shown in Fig. 87 being typical. This particular shunt was 
introduced by the Western Electric Company in the early days of 
telephony. Referring to this figure, a and e represent the terminals 
of the generator, the path between them through the armature being 
traced from the terminal, a, through the lower portion of the spring, 
b, which bears against the armature pin, thence through the wind- 
ing, /, of the armature and the frame of the generator to the ter- 
minal, c. When a generator is not being operated, however, a low- 
resistance shunt exists around the armature which may be traced 
from the terminal, a, through the upper portion of spring, b, which 
rests against the end of the shaft, s, which in turn is in contact 
with the terminal, e, through the bearing plates, c and d, to the frame 
of the machine. 



118 AMERICAN TELEPHONE PRACTICE. 

The large gear-wheel, G, is loosely mounted on the shaft, s, the 
shaft passing through a notched collar, forming the hub q£ the gear. 
The rotary motion of the shaft is transmitted to the gear-wheel by 
means of the pin, p, projecting from the shaft and engaging the 
notch in the collar. A coiled spring surrounding the shaft, pressing 
at one end against the shaft bearing, d, and at the other against a 
collar fixed to the shaft, serves to press the shaft normally to the 
left into contact with the spring, b. A coiled spring on the shaft 
also serves to keep the pin, p, normally at the bottom of the notch 
in the hub of the gear-wheel. 

When th generator crank is turned, the pin, p, rides partially out 
of the notch in the hub of the gear-wheel, and in so doing pulls the 




FIG. 88.-HOLTZER-CABOT THREE-BAR GENERATOR. 

shaft out of contact with the spring, b, thus breaking the shunt 
around the armature and leaving the latter effectively in the line. 

This is only one of many forms of automatic shunting devices, 
some of which will be shown in connection with commercial forms 
of magneto generators. 

Instead of shunting the generator armature when not in use, some 
forms of telephone service require that it shall be left on open cir- 
cuit when not in use. Such a device is called a "generator cut-in," 
and is almost always automatically operated by the turning of the 
crank, although, of course, they may be manually operated by a 
button. These devices differ from the automatic shunts only in 
that they serve to normally hold the armature circuit open and to 
close it when the generator is operated, instead of serving to break 
the circuit around the armature when operated. 



MAGNETO CALLING APPARATUS. 



119 



Coming now to commercial types of magneto calling apparatus, 
the Holtzer-Cabot Company, of Brookline, Mass., are manufactur- 
ing an excellent magneto generator, two types of which are shown 
in Figs. 88 and 89. 

The end plates in which the generator armature is journaled are 
of cast brass and are secured directly to the cast iron pole-pieces, 




FIG. 



-HOLTZER-CABOT FOUR-BAR GENERATOR. 



the ends of which are flanged and machined so as to fit accurately 
the internal curves of the pole-pieces. The armature core is of soft 
iron laminations, the layers being in the form of punchings clamped 
together on a steel rod which therefore serves as the armature shaft. 
The construction of the armature and the shape of one of the punch- 
ings or laminations is clearly shown in Fig. 90. After being built 



FIG. 




LAMINATED ARMATURE. 



up in this manner, the core is properly insulated with cloth saturated 
with shellac, after which the winding is put on. 

The most common method of transmitting the motion of the 
crank to the armature shaft is by means of a large gear-wheel on 
the crank shaft, meshing with a pinion on the armature shaft, the 
ratio of speed in rotation between the two shafts being about as 



120 



AMERICAN TELEPHONE PRACTICE. 



one to five. The Holtzer-Cabot Company introduced, and once 
used to a large extent, a chain-drive, by means of which a chain, 
having links of steel wire passed over two sprockets, as shown in 
Fig. 91, in practically the same manner as in the ordinary chain- 
driven bicycle. This afforded a very smooth-running, quiet drive, 




FIG. 91.— CHAIN-DRIVEN MAGNETO GENERATOR. 

but this has been almost entirely superseded by the direct meshing 
spur gears, a detail of which is shown in Fig. 92. In this, both the 
large gear and pinion are of brass, having wide faces cut to secure 
smooth running. In order to prevent the jerk occasioned by the 




FIG. 92.— SPUR GEARING FOR GENERATORS. 

magnetic pull of the field on the armature, with its consequent back- 
lash, the pinion, instead of being rigidly secured to the shaft, has a 
flexible connection therewith. This consists of a coiled spring 
within the pinion, being secured at one end to the armature shaft 
and at the other to the pinion itself, so that whatever motion is trans- 



MAGNETO CALLING APPARATUS. 121 

mitted from the pinion to the armature shaft is through the medium 
of the spring. This point illustrates one of the niceties of detail 
now required by the telephone-using public. 

The shunt used to a large extent by the Holtzer-Cabot Company 
is shown in Fig. 93. It is an ingenious little device consisting of 
a small cylindrical case or cup of brass mounted directly on the 
projecting portion of the armature shaft, and in metallic contact 
therewith. One terminal of the armature winding is therefore con- 
nected through the armature core to this cup. The insulating pin 
with which the other terminal of the armature winding is connected 
projects into the chamber formed by this case; but does not come 
into metallic contact with the casing nor the armature shaft itself. 
The chamber is partially filled with small bits of metallic wire which 
normally form a connection between the central pin and the casing 
itself, thereby forming a short circuit or shunt around the armature 
winding. When, however, the armature is rotated, the centrifugal 




FIG. 93.— HOLTZER-CABOT SHUNT. 

force, due to the rotation acting upon the bits of wire, causes them 
to fly to the outer portion of the casing, thus breaking contact with 
the central pin and removing the shunt from the armature. This 
is probably the simplest shunt on the market, and is reliable. In 
order to prevent corrosion of the parts, the casing and the small 
particles of wire are silver-plated. The cut at the left of Fig. 93 
represents the condition of affairs when the armature is at rest, and 
that at the right when it is in use. 

Another form of shunt used on the generators of the Holtzer- 
Cabot Company is shown on the type of generator of Fig. 89. 
already described. In these, by means of a device similar to that 
shown in Fig. 87, the crank shaft is automatically given a longitu- 
dinal movement to the left when operated. This presses the long- 
spring which bears against the insulated end of this crank shaft out 
of contact with the right-hand spring of the group of three with 
which it normally makes contact, and into engagement with the left- 



122 



AMERICAN TELEPHONE PRACTICE. 



hand spring of this group with which it is normally out of contact. 
It is evident, by using the long spring and the right-hand spring 
only, that this device may be used as a generator shunt, while if the 
long spring and the left-hand spring only are used, it may be used 
as a generator cut-in. 

Fig. 94 is of interest as showing, in side and front elevation, a 
form of generator now obsolete, but once manufactured by the 
Williams-Abbott Company, of Cleveland. This is one of those 
generators in which an attempt was made to use pole-pieces formed 
from sheet iron instead of making them of cast iron, as is now done. 
Inspection of the end elevation of this figure will show a poor mag- 
netic path between the ends of the terminal magnets and the pole- 




FIG. 94.— WILLIAMS-ABBOTT GENERATOR. 

pieces. The only direct path is through the line of contact, as at A, 
another path being afforded down through the thin cross section 
of the metal forming the lower portions of the pole-pieces. 

In Fig. 95 are shown the parts of a generator recently put on 
the market by the Kellogg Switchboard and Supply Company, this 
being built along standard lines with the exception of the method 
of constructing the armature, which, if not new. is at least a radical 
departure from modern practice. Instead of forming the shaft 
integral with the armature core, the two ends of the shaft are sepa- 
rate, and are secured to the ends of the armature core by means of 
flanges properly turned and fitted and secured in place by screws. 
The armature core itself is of cast iron and is provided with a channel 
throughout its length on both sides to afford space for the winding. 



MAGNETO CALLING APPARATUS. 



123 



Under this construction the shaft is not continuous and therefore 
does not interfere with the winding space, as in the usual armature 
construction. As a result, more winding space is available, which 




w^ 



FIG. 95.— PARTS OF KELLOGG GENERATOR. 

may be used in obtaining more turns in case high voltage is neces- 
sary, or in obtaining more current in case that is necessary. The 




FIG. 9U.-KELLOGG GENERATOR PARTLY ASSEMBLED. 

outcome of this form of armature in practice will be watched with 
interest. 

This generator is shown partly assembled in Fig. 96, the magnets 
only being removed. This figure shows clearly the method of 



124 



AMERICAN TELEPHONE PRACTICE. 



operating the shunt or cut-in, this being of the same type as in the 
Holtzer-Cabot generator of Fig. 89. 

The standard ringer used by the Hoitzer-Cabot Company is 
shown in Fig. 97. 

Another view, showing also the method of mounting it in proper 




FIG. 97.— HOLTZER-CABOT RINGER. 

relation with the gongs, is shown in Fig. 98. This general con- 
struction may be considered standard, and has given long-continued 
satisfaction in use. The gongs are mounted on adjustable stand- 
ards pivoted at their upper ends and each held by a screw engaging 
a slot in its lower end. This ringer differs from that shown in Fig. 
83 in that the yoke in which the armature is pivoted is of heavy, 



FIG 




HOLTZER-CABOT RINGER. 



spring brass, its ends being fitted over the pole-pieces of the mag- 
nets which project slightly through it. The adjustment of the 
armature toward or from the pole-pieces is effected by means of a 
screw, best shown in Fig. 97, this screw being threaded into the 



center of the yoke piece and passim 



freely through a hole 



n the 



MAGNETO CALLING APPARATUS. 



125 



armature, its head engaging the under surface of the permanent 
magnet. Upon turning this screw in a left-handed direction with 
the head bearing against the under side of the permanent magnet, 
the center portion of the yoke is forced downward against its 
natural tendency, thus allowing the ends of the armature to ap- 
proach more nearly the pole-pieces. Turning the screw in the 
other direction allows the spring of the yoke to force the armature 
further from the pole-pieces, thus securing a coarser adjustment. 

Fig. 99 shows two views of the working parts of the Stromberg- 
Carlson ringer. This is similar to that of the Holtzer-Cabot 
Company, save for the method of adjustment. In this, the adjust- 
ment screw, B, projects entirely through the turns freely in the end 
of the magnet, A. The armature, C, is carried by the spring yoke, 
D, which fits over the pole-pieces, E E, which are held together by 





FIG. 99.— STROMBERG-CARLSON RINGER. 



a separate yoke, F. The screw, B, passes freely through the arma- 
ture and engages by a shoulder, as shown, the spring yoke, D, 
through which it also passes, being threaded into the yoke, F. It 
is evident that turning the screw in one direction or the other will 
press the center of the yoke carrying the armature toward the yoke 
F, or allow it to spring further away from it, thus effecting the 
adjustment between the armature and the pole-pieces. 

Fig. ioo shows an excellent form of ringer designed by Mr. E. E. 
Yaxley, and now being extensively manufactured by the Monarch 
Telephone Manufacturing Company, of Chicago. This differs from 
the old Western Electric ringer in the method of supporting the 
armature yoke. Instead of supporting this on separate rods out- 
side of the coils as shown at D D, in Fig. 83, Mr. Yaxley provides 
the free end of his magnet cores with screw threads, upon each of 



126 



AMERICAN TELEPHONE PRACTICE. 



which fit two hexagonal nuts which carry between them the arma- 
ture yoke. This is, therefore, the same form of adjustment as used 
in the Western Electric ringer save that the cores, instead of the 
extra rods, are made to serve as parts of the frame carrying the 
armature yoke. 

The new ringer of the Kellogg Switchboard & Supply Company 
is shown in Figs. 101 and 102. This possesses two features not 
shown in any device so far considered. Perhaps the most important 
of these is that the whole ringer, including the gong and gong sup- 




FIG. 100.-YAXLEY RINGER. 



ports, is self-contained, the ringer being a complete entity regard- 
less of the front board of the telephone upon which the various 
parts are usually separately mounted. In this the gong posts are 
mounted on levers pivoted at one end on the heel-piece, upon which 
the magnets are also fastened. Screws passing through the slotted 
portions of these levers serve to allow, when loosened, a certain 
longitudinal motion of the gongs with respect to the clapper and 
to bind them firmly into position when tightened. The other 
unique feature about this ringer is that adjustment is effected by 
carrying the pole-piece toward or from the armature rather than 
moving the armature with respect to the pole-piece. The pole- 
pieces are, in fact, hexagonal screws of iron engaging internal 



MAGNETO CALLING APPARATUS. 



127 



threads in the cores of the magnets, these screws being moved to- 
ward or from the armature in order to secure a fine or coarse 




FIG. 101.— PARTS OF KELLOGG RINGER. 

adjustment. A hexagonal nut is carried on each screw in order to 
lock the screw in any desired position. 




FIG. 102.— KELLOGG RINGER ASSEMBLED. 



The ringer shown in Fig. 103 is unique and embodies several de- 
sirable features, although it has become obsolete. The two ends of 



128 



AMERICAN TELEPHONE PRACTICE. 



the U-shapecl permanent magnet are of the same polarity, its 
middle being of the opposite polarity. The two coils are mounted 
on an iron cross-bar extending between the legs of this permanent 
magnet. The poles of the electro-magnets therefore partake of the 
polarity of the permanent magnet ends, while the armature, sup- 
ported from the center of the permanent magnet, becomes of the 
opposite polarity. The action of this ringer is usually misunder- 
stood at first sight, the natural supposition being that the limbs of 
the permanent magnet are of opposite polarity. This ringer is very 
efficient, and it is surprising that it should not have remained in 
service, perhaps in a somewhat modified form. Besides being de- 
sirable in point of efficiency, it has the advantage of having its 
working parts partially enclosed by the rigid permanent magnet, 
which serves to protect them from mechanical injury. 

All of the generators so far described have been adapted to give 




FIG. 103.— OLD WILLIAMS-ABBOTT RINGER. 



only alternating currents, the current consisting of a positive and 
negative impulse for every complete revolution of the armature. 
It is sometimes desirable to make all of the impulses in the same 
direction, as in certain classes of party line ringer, and for this pur- 
pose a form of commutator is sometimes applied to the end of the 
armature shaft which will rectify current reversing every alternate 
impulse. Such current is frequently spoken of as "direct" current, 
but more often as "pulsating" current, and a generator provided 
with this device is termed a "direct current" or "pulsating current" 
magneto generator. 

In party line working it is often necessary to make ringers that 
will respond to only one direction of current; that is, to impulses 
of one direction only, whether positive or negative. This is accom- 
plished by adding to the ordinary ringer a little coiled spring 



MAGNETO CALLING APPARATUS. 



129 



tending to hold the armature normally in engagement with one pole 
or the other. This is quite clearly illustrated in principle in Fig. 
104. Various methods of adjusting the tension of this spring have 




FIG. 104.-DIAGRAM OF BIASED BELL. 



been put into practice, probably the best form being that wherein 
a linen or silk thread tied to the free end of the spring is wound 
around a screw in a similar manner to the method used in adjusting 




FIG. 105.-MODERN BIASED BELL. 

Morse relays. Such an adjustment as applied to a modern ringer 
is shown in Fig. 105. 

A ringer equipped for this purpose is usually termed a "biased"' 
bell or "biased" ringer. It is evident that when the current im- 
pulses are in such direction as to attract that end of the armature 
9 



130 AMERICAN TELEPHONE PRACTICE. 

which is already held near its pole-piece by the spring, no further 
effect will take place. If, however, the current is in opposite direc- 
tion, it will cause the opposite end of the armature to be attracted 
to its pole-piece against the tension of the spring, thus causing the 
bells to ring. Biased bells require very careful adjustment and are 
not altogether satisfactory, yet, as will be described later, they are 
widely used in certain classes of work. 






CHAPTER IX. 

LOCAL BATTERY SUB-STATION EQUIPMENTS. 

So far talking apparatus and calling apparatus have been consid- 
ered separately. When both talking and calling apparatus are 
properly combined for meeting the requirements of actual service, 
it is called a complete telephone set, or a sub-station equipment. 
The term, "sub-station," probably an abbreviation of "subscriber's 
station," is now commonly used to designate the premises of any 
telephone user or subscriber, hence the term sub-station equipment. 

Sub-station equipments may be classified under two heads in ac- 
cordance with the kind of system with which they are adapted to 
be used; thus we may have local battery sub-station equipments, 
also termed magneto sub-station equipments, on account of the 
magneto generator being usually found in sets of this kind ; or, we 
may have common battery or central energy sub-station equip- 
ments, in which no local battery or magneto generator is found. 
Only local battery or magneto sub-station apparatus will be con- 
sidered in this chapter, as that for common battery systems may be 
better understood after a discussion of common battery systems in 
a subsequent chapter. 

Local battery sub-station apparatus may in turn be classified 
under two heads, series and bridging. The series type of telephone 
is that wherein the arrangement is such as would be used were a 
number of instruments placed in series in the same circuit, a prac- 
tice no longer very common. They are, as has been before stated, 
usually provided with a comparatively low resistance ringer magnet 
and with a generator having its armature normally shunted. The 
instruments of the bridging type are such as are adapted to be 
placed in multiple or bridge relation with the line, as is commonly 
done in lines having more than one sub-station. The bridging 
telephone set should be provided with a ringer magnet of high re- 
sistance and retardation, the generator being very powerful and 
normally on open circuit instead of being shunted, as in series in- 
struments. 

It is obvious that as the calling and talking apparatuses of a sub- 
station equipment are used alternately, some means is necessary 

lfl 



132 



AMERICAN TELEPHONE PRACTICE. 



for switching one or the other into the circuit. An instrument 
when not in use must always be ready to receive a call, and the call 
bell or ringer must therefore normally be left in the circuit of the 
line. Furthermore, as the retardation of the ringer coils would be 
in many cases detrimental to the transmission of talking currents, 
this coil must in such cases be switched out of the circuit of the line 
when the talking instruments are in use. 

At first, hand switches were used to accomplish this result. Before 
the adoption of the battery transmitter, instruments were provided 
with ordinary two-point hand switches, so arranged as to alternately 
close the line circuit through either of two branches, one containing 
the call bell and generator and the other the magneto telephone. 
Such an arrangement is shown in Fig. 106, where a is a switch lever 
pivoted at one end and adapted with its other end to make contact 




FIG. 106.— HAND-SWITCH CIRCUIT. 



with either of the metallic buttons, 1 or 2. One side of the line 
circuit is connected to the pivot of the lever, a, while button No. 1 
is connected through the call bell, b, and generator, g, to the other 
side of the circuit. Thus, when the switch lever is in the position 
shown, the call bell and generator only are in the circuit. Between 
the button 2 and one side of the line circuit is attached the magneto- 
telephone, /, so that when the lever, a, is in the position shown in 
dotted lines, the circuit between the two sides of the line is com- 
pleted through this telephone, the calling apparatus being out of 
circuit. 

This manual operation of the switch proved very unsatisfactory. 
People would continually forget to move the switch lever into its 
normal position in contact with button No. 1, after using the tele- 
phone, and thus would leave their sub-station apparatus in such 
condition as to render it impossible to receive a call. It was soon 



LOCAL BATTERY SUB-STATION EQUIPMENTS. 



133 



after found necessary to make the switch as nearly automatic as 
possible, and to attain this end the switch lever was so designed as 
to be held down by the weight of the receiver in contact with the 
terminal of the calling circuit, and when released therefrom by the 
removal of the receiver for use to be moved by a spring into contact 
with the talking circuit terminal. Soon after, battery transmitters 
having come into general use, it became necessary to provide means 
for opening and closing a local circuit containing the local battery, 
the primary of the induction coil, and the variable resistance trans- 
mitter. This was necessary in order to have the battery in use only 
when the telephone instrument was being used, and was accom- 
plished by the addition of a single contact point with which the 
lever made contact when released from the weight of the receiver. 
In order to make the lever a convenient place on which to hang 



artt^ 




FIG. 107.-SERIES CIRCUIT-HOOK DOWN. 



the receiver, its free end projected from the side of the instrument 
within convenient reach of the user, and was provided usually with 
a forked hook upon which the receiver might be supported. It is 
from this hook that the name hook-switch or switch-hook has been 
derived. 

Fig. 107 shows, in somewhat modified form, the circuit of an 
ordinary magneto or local battery instrument. In this the hook 
lever is represented by H, and as will be seen, it is adapted to 
vibrate between the contacts 1 and 2 in its raised position and 
contact 3 in its depressed position. A restoring spring, not shown, 
serves to lift the lever into such a position that it will make contact 
with the points 1 and 2 when released from the weight of the re- 
ceiver, while, when the receiver is hung on the fork of the hook 



134 



AMERICAN TELEPHONE PRACTICE. 



this spring is overcome, the lever being brought into contact with 
the point 3. In its normal position, that is, when the receiver is 
hung up, all talking circuits are inoperative, being open at the points 
1 and 2, and are therefore, for the sake of clearness, represented in 
the figure by dotted lines, those parts of the circuits which are in 
operative relation with the line being represented by full lines. 
Under these conditions a calling current from some other station 
coming in over the line to the binding post, L, forming one terminal 
of the instrument, will pass through wire, a, the shunt, s\ of the 
generator, G, thence through the winding of the call bell magnet, C, 
to contact, 3, and through the lever to the binding post, L', con- 
nected to the return side of the circuit. When the instrument is 
used for sending a call, the generator crank is turned, automatically 




FIG. 108.— SERIES CIRCUIT— HOOK UP. 

breaking the shunt around the armature, and thereby allowing the 
current generated in the armature to pass out over the line and ring 
the bell at a distant station. 

In Fig. 108 the hook is shown in its raised position as when re- 
leased from the weight of the receiver. The breaking of the contact, 
3, renders the generator and bell inoperative, and therefore the cir- 
cuit through them in this figure is shown in dotted lines. The 
closing of the contacts 1 and 2 by the raising of the hook puts the 
talking apparatus, including the receiver, R, the battery, B, the 
transmitter, T, and the primary and secondary, p and s, respectively, 
of the induction coil in operative relation with the line. The bat- 
tery, transmitter and primary winding of the induction coil are in a 
local circuit between the contacts 1 and 2, while the line circuit is 
closed through the receiver and secondary winding of the induction 



LOCAL BATTERY SUB-STATION EQUIPMENTS. 



135 



coil and the contact, 2, of the hook-switch. In transmitting, undula- 
tory currents set up in the local circuit will be transformed into 
alternating currents in the secondary of the induction coil, which 
currents will pass out over the line circuit and through the receiver 
and secondary coil of the distant receiving station, provided, of 
course, the hook at that station is also in its raised position. 

The arrangement shown in Figs. 107 and 108, just described, is 
that of the ordinary local battery or magneto-telephone adapted to 
series work in common use to-day. The hook lever alternately 
renders inoperative either the talking apparatus or the calling ap- 
paratus by opening their circuits. Another method has been pro- 
posed and used to some extent, although it is now not in common 




FIG. 109.— SERIES SHUNTING CIRCUIT— HOOK DOWN. 



use. It is to have the hook lever render the talking apparatus or 
the calling apparatus inoperative by short-circuiting them instead of 
opening their circuits. Such an arrangement is shown in Figs. 109 
and no, where it will be seen that when the hook is down the gen- 
erator, G, and the call bell, C, are in the proper circuit relation with 
the line for use, while the receiver and secondary winding of the 
induction coil are short-circuited by the closure of the contact, 3. 
Thus a calling current either outgoing or incoming would pass 
through the calling apparatus to the hook and thence through the 
shunt wire, a, around the receiver and secondary winding, those be- 
ing shunted out of the circuit by the low resistance of this wire. On 
the other hand, when the hook is raised, as shown in Fig. no, the 



136 



AMERICAN TELEPHOXE PRACTICE. 



shunt wire, a, is broken at the contact, 3, thus placing the receiver 
and secondary winding in proper relation with the line for use, 
while the shunt path, b, closed at the contact, 1, serves to direct the 
voice currents around the generator and call bell, thus shunting 
them out of circuit. The local circuit is closed and opened with this 
arrangement at the contact, 2, in the same manner as shown in Figs. 
107 and 108. This shunting arrangement has an advantage over 
the other in that bad contact in the switch-hook does not com- 
pletely spoil the operation of the instrument. If, for instance, the 
hook does not make the proper contact or makes no contact at 
all when depressed against point, 3, the calling apparatus would 




FIG. 110.— SERIES SHUNTING CIRCUIT— HOOK UP. 

still be effective, as a path for the calling currents would still exist 
through the receiver and secondary winding of the induction coil. 

In Figs, in and 112 are shown two circuits representing in sim- 
plified form the arrangement of apparatus commonly used in bridg- 
ing telephones, the first figure showing the condition of affairs 
when the hook-switch is depressed for receiving or sending a call, 
and the second showing the condition for talking when the hook 
is raised. As in the preceding figures, the apparatus and circuits 
rendered inoperative by the position of the hook-switch are shown 
in dotted lines. Referring to Fig. in, where the receiver is repre- 
sented upon its hook, it will be seen that the talking apparatus is all 
on open circuit. The bell, C, is, however, permanently bridged 
across the line, its circuit in relation with the line depending in 



LOCAL BATTERY SUB-STATION EQUIPMENTS. 



137 



nowise on the position of the hook. The generator, G, is likewise 
independent of the hook in regard to its connection with the line, 
but is normally on open circuit at the point, a, which is closed only 
while the generator is being operated. The call bell, C, is adapted 
to receive incoming calling currents in an obvious manner, it being 
the only path closed across the two sides of the line circuit. The 




FIG. 111.— BRIDGING CIRCUIT— HOOK DOWN. 



generator, G, when operated Automatically closes the contact, a, and 
sends current out to the line past the terminals of the call bell, C 
Where used on party lines the call bells of all the stations on the 
line are placed in multiple and are therefore all responsive to ring- 
ing current sent over the line. 




FIG. 112.— BRIDGING CIRCUIT— HOOK UP. 



When the hook is raised, as shown in Fig. 112, the talking ap- 
paratus is properly connected with the line for use, in the manner 
already described, but the condition as to the generator and ringer 
is not altered. The generator circuit is, however, open, so that its 
circuit need not be considered as affecting the voice transmission. 
The bridge across the talking circuit afforded by the presence of the 



138 



AMERICAN TELEPHONE PRACTICE. 



ringer magnets, C, would at first be thought detrimental to the 
talking efficiency, as it apparently affords a leak through which a 
portion of the voice currents may pass. The very high retardation 
of the ringer magnets, however, effectually prevents this, and it is 
found in practice that the presence of one or a comparatively large 
number of these magnets across the line does not affect the voice 
transmission. 

The diagrams of circuits so far shown were somewhat modified 
/ 3 Jt 




FIG. 113.— CIRCUIT OF COMPLETE SERIES TELEPHONE— COIL IN 
BASE OF ARM. 

in order to render clearer an understanding of the working of the 
circuits. In Fig. 113 are shown the circuits of a complete series 
local battery telephone set of the type shown in Figs. 107 and 108, 
the connections being arranged as in actual practice. It is cus- 
tomary to mount the generator, G, the polarized bell, P, the hook- 
switch, L, and sometimes the induction coil, /, all in one box, as 
shown in Fig. 114, such a box with its contents being commonly 
known as a magneto-bell box. Frequently the induction coil is 
omitted from the box, as is the case in this figure. When this is 



LOCAL BATTERY SUB-STATION EQUIPMENTS. 



139 



done the induction coil is mounted in the base of the transmitter 
arm. The circuit of Fig. 113 is adapted to such arrangement. 

In order to facilitate the work of making connections between 
the parts contained in the generator box and the other parts, that 
is, the transmitter, receiver, battery and induction coil, in case this 
is not placed within the box, and also with the line, the terminals 
of the circuit in the box are brought out to binding posts on the 
top and bottom of the box. In Fig. 113 the binding posts 1 and 2 
on the top of the box are for attaching the line wires. These form 




FIG. 114.— MAGNETO BELL BOX. 

the terminals of the instrument as a whole. The center binding 
post forms the ground terminal of the lightning arrester, not shown 
in this figure, and has no connection within the box. 

The six binding posts at the bottom of the generator box are in 
three pairs, r, /, and s. The pair, r, form terminals for attaching 
the receiver cord. The two posts forming pair /. are for the local 
circuit, the battery, B, the transmitter, T, and the primary of the 
induction coil, I, being connected in series between them. Between 
the right-hand pair of posts, s, is connected the secondary of the 
induction coil, /. The bell, P, is mounted on the door of the. box, 
connection to it beincr made throuqh the hincres. 



140 



AMERICAN TELEPHOXE PRACTICE. 



The connections of the automatic shunt are clearly shown in this 
figure. Normally a short circuit exists around the generator, 
through the wire, s. This is- broken between the pair of springs 
when the generator is operated as already described. 

In Fig. 115 is shown a view of a complete instrument using the 
circuits just described, this type of telephone being well known and, 
until recently, being almost the only form in which magneto tele- 
phones for wall use were produced. In this type of instrument the 




FIG. 115.— COMPLETE WALL TELEPHONE SET. 



battery, usually consisting of two wet cells of the LeClanche type, 
is located in the box at the bottom of the wall board, the battery 
resting on a cast iron shelf secured to the wall board and being 
completely exposed for inspection by the removal of the box. 
Various methods of fastening the box have been used, but one, 
consisting of two clamps on the inner side of the battery box 
adapted to engage two similar clamps secured to the back board, 
has come into wide use. The box is engaged or disengaged by a 
downward or upward motion with respect to the back board. 



LOCAL BATTERY SUB-STATION EQUIPMENTS. 



141 



The complete circuits of the bridging belF telephone set are 
shown in Fig. 116. This shows the arrangement used in practice, 
embodying the simplified circuits shown in Figs, in and 112. It 
will be seen that the bell, P, is permanently bridged across the two 
sides of the line between the binding posts 1 and 2, the connections 
being made through the hinges of the bell box.' The generator, G, 
is in a second bridge circuit, normally open, but adapted to be. 
closed when the generator is operated. 



/ ? & 




FIG. 116.— COMPLETE BRIDGING BELL TELEPHONE CIRCUIT— COIL 

IN BASE OF ARM. 



In Fig. 117 are shown the complete working circuits of a series 
telephone wall set in which the induction coil is mounted in the bell 
box rather than in the base of the transmitter arm. Fig. 1 18 shows 
the circuits of a bridging telephone using a similar arrangement. 
This practice of mounting the induction coil in the bell box rather 
than in the base of the transmitter arm is probably, all things con- 
sidered, the best. There is no good reason why the coil should be 
mounted in the base of the transmitter arm and the circuit wiring 



142 



AMERICAN TELEPHONE PRACTICE. 



of the box is made somewhat simpler by mounting the coil in the 
bell box. 

Since the use of the dry cell has become very common for local 
battery telephones, a new type of wall set has come into vogue 
wherein the call bell, generator, switch-hook, induction coil and 
batteries are all mounted in a single box divided off into compart- 
ments, as shown in Fig. 119. Another view of this box with the 
door closed is shown in Fig. 120. This type of set is being made 




FIG. 117.— SERIES CIRCUIT— COIL IN BELL BOX. 

by nearly all of the telephone manufacturers and is proving popular. 
It is perhaps more convenient, and neater in appearance than in 
the old types wherein a large battery box is provided for the wet 
cells. In this latter type a shelf for the convenience of the sub- 
scriber in writing is mounted on the lower part of the cover of the 
box instead of using the top of the battery box for this purpose, as 
in the old type. 

The arrangement of transmitter, receiver, induction coil and 



LOCAL BATTERY SUB-STATION EQUIPMENTS. 



143 



hook-switch shown in Fig. 121 deserves mention. It is that used 
by the Stromberg-Carlson Company, which company has per- 
sistently adhered to this type for a large portion of their local bat- 
tery and common battery telephones, for several years past. In 
this the hook-switch and induction coil are both mounted in the 
base of the transmitter arm, this base being a hollow box of cast 
iron in the form shown. The hook lever projects through the open- 
ing in the left-hand portion of this box, for supporting the receiver. 




FIG. 118.— BRIDGING CIRCUIT— COIL IN BELL BOX. 



All the wiring between the terminals of the receiver, transmitter, 
induction coil and hook-switch is concealed within this box, the 
terminals of their combined circuit being brought out on binding 
posts on a connecting rack at the top of this box. This piece of 
apparatus therefore contains all of the talking apparatus of the 
ordinary telephone; and it is only necessary to properly connect it 
in circuit with the generator, ringer and battery in order to obtain 



144 



AMERICAN TELEPHONE PRACTICE. 



a complete magneto sub-station apparatus. Such a set in its com- 
plete form is shown in Fig. 122. 

The so-called "desk set" has become more and more popular, 
particularly in the case of busy men who are compelled to use their 
telephones many times a day. Desk sets are usually made in this 
country of the general type shown in Fig. 123, although the various 
manufacturers have modified the design in many details in order 
to meet the demands of trade. In the case of magneto-desk sets, 
it is customary to mount the magneto generator and ringer sepa- 
rately from the desk stand, sometimes in two separate boxes. The 




FIG. 119.-DRY-BATTERY WALL SET-CLOSED. 



battery is usually arranged in a drawer of the desk or on the floor 
at some convenient point near the desk. The arrangement of 
circuits in a desk set involves several considerations not found in 
the circuit of a wall set. The wall set is usually complete in itself, 
so that the wiring may be completed in the factory, the only wires 
that need be applied to the instrument externally being those lead- 
ing to the line and ground. In the desk set, the separate mounting 
of the generator, the ringer and the battery, makes their connection 
in their proper circuit relation, without an undue amount of wiring 
(the presence of which would be an inconvenience to the user of the 
desk) a matter which requires careful study. In order to make the 



LOCAL BATTERY SUB-STATION EQUIPMENTS. 



145 



desk stand proper portable it is usual to connect it to the other 
parts of the circuit by a flexible cord having a sufficient number 
of separate conductors. This cord usually has an outer braiding 




FIG. 120.— DRY-BATTERY WALL SET— OPEN. 

of green silk and is long enough to give sufficient range of motion 
to the desk stand. The number of conductors in this cord is a 




FIG. 121.— STROMBERG-CARLSON "TRIPLET. 



matter which should be given some weight in laying out the scheme 
of wiring, as the cord is an item of no inconsiderable expense, and 
on general principles, the fewer the number of conductors in a 

10 



146 



AMERICAN TELEPHONE PRACTICE. 



flexible cord the better. As an aid to desk set wiring it is not un- 
common to use a separate connecting rack having several binding 
posts which may be mounted in any convenient part of the desk 
and to which the various wires leading to the generator, bell, bat- 
tery, etc., may be attached. Fig. 124 shows the general scheme of 
wiring used by the Stromberg-Carlson Company in their series 
magneto desk set arrangement, the generator being mounted in the 
box on which the desk stand rests and forming a sort of shelf there- 
for if desired. In case it is wished to keep the desk stand on the 
desk proper, the generator box may be mounted under or beside 




FIG. 122.-STROMBERG-CARLSON TELEPHONE USING '"TRIPLET. 



the desk so that its crank may be in easy reach of the subscriber. 
This shows the standard Stromberg-Carlson desk stand, in which 
the induction coil is carried in the base of the stand. 

In Fig. 125 is shown a different arrangement, wherein the induc- 
tion coil is mounted on the connecting rack, and the ringer is 
enclosed in the generator box. This cut shows the standard 
Kellogg desk stand. 

In Figs. 126 and 127 are shown respectively the circuits of series 
and bridging desk sets, the apparatus being so arranged that the 
induction coil, generator and bell are all within a single box, bind- 



LOCAL BATTERY SUB-STATION EQUIPMENTS. 



147 



ing posts on the base of this box serving as terminals for the 
attachment of the flexible cord leading to the desk stand proper, as 
well as the battery terminals and the line terminals. In this case, 
which is probably the simplest of all magneto desk set arrange- 
ments, a four-conductor cord from the desk stand to the generator 
box is required for the series set, a three-conductor cord serving 
for the bridging instrument. Besides this the only external 




FIG. 123.— TYPICAL DESK STAND. 



wiring necessary is that from the line and the battery to the posts 
on the base of the box. In Figs. 128 and 129 are shown similar 
circuits for series and bridging instruments respectively, the differ- 
ence in this case being that a separate connecting rack is employed 
upon which the induction coil is mounted, the magneto box in this 
case containing only the generator and the call bell. In these a 
three or four-strand flexible cord is needed from the connecting rack 
to the desk stand; two wires lead from the connecting: rack to the 



148 



AMERICAN TELEPHONE PRACTICE. 



bell box, two from the connecting rack to the battery and two from 
the connecting rack to the line. This arrangement is advantageous 




FIG. 124.— WIRING OF DESK SET. 



where, for any reason, it is undesirable to run a flexible cord leading 
from the desk stand to the generator box. By mounting the in- 




desk stand \ 



m 





FIG. 125.— WIRING OF DESK SET. 



duction coil on the connecting rack, the four wires leading to the 
primary and secondary of this coil may be connected permanently 



LOCAL BATTERY SUB-STATION EQUIPMENTS. 



149 



to their respective binding posts on this rack, thus somewhat sim- 
plifying the external wiring. 

Still another arrangement of desk stand wiring is that wherein 
a separate connecting rack without induction coil is used, the in- 
duction coil being carried in the generator box. The circuit ar- 
rangement for this or any other modification may easily be worked 
out from the diagrams shown. 

For bridging instruments, the Bell companies usually employ the 
circuit arrangement shown in simplified form in Figs, in and 112, 



4-Conductor 
Ccrd-ZT' 




3at~ 



FIG. 126.-SERIES DESK SET CIRCUIT-COIL IN BELL BOX. 



the generator and bell being independent of the position of the 
hook. This was the arrangement of the Carty patent. Several 
modifications of this arrangement are used by different Independent 
companies, these owing their existence either to the demand for 
slightly different kind of service or directly to a desire to dodge the 
claims of that patent. 

The simplest of these modifications is shown in Fig. 130. where 
both generator and bell are rendered incapable of sending current 
to, or receiving it from, the line when the hook is raised. 



150 



AMERICAN TELEPHONE PRACTICE. 



In Fig. 131 the bell circuit only is under the control of the hook, 
the generator being connected directly across the binding posts of 
the instrument at all times. 

In Fig. 132 the circuits of both generator and bell are controlled 
at the hook, and furthermore the circuit of the bell, normally closed, 
is opened by the action of the generator, which at the same time 
cuts itself into the circuit of the line. With this arrangement the 




® ® ® 

3 Conductor V 
Cord^ 



_— J5&f.~* 



FIG. 127.— BRIDGING DESK SET CIRCUIT— COIL IN BELL BOX. 



generator can never cause its own bell to ring, a feature desired by 
many users. 

The apparatus by which the automatic switching between the 
signalling and the talking apparatus is accomplished — the switch- 
hook — while very simple, merits attention in detail. Much care is 
necessary in its design and construction. The energy available for 
the operation of the switch is limited to that due to the attraction 
of gravity on the receiver, and it becomes somewhat difficult to so 
arrange the contacts that they will be firmly and positively made, 
and surely broken at the proper time. For this reason all the points 



LOCAL BATTERY SUB-STATION EQUIPMENTS. 



151 



of contact are in all good types provided with platinum tips to pre- 
vent corrosion, and, if possible, a slight sliding action at the point of 
contact is also obtained. A sliding contact tends to clean the 
points and at the same time prevent particles of dust from keeping 
the two apart. Too much sliding action is, however, worse than 
none, as it is sure to cause cutting. The springs for restoring the 
lever and those serving as contacts should be so arranged that no 
movement of which the lever is capable will strain them beyond 



■4- Conductor 
OH'd-^ 



— *H 



)■ De3k $land '_ 




FIG. 128.— SERIES DESK SET CIRCUIT— COIL ON RACK. 



their elastic limit or to such a degree that they will eventually lose 
their tension or break. It is bad practice to have the same part of 
a contact slide alternately over a conducting material, as of brass. 
and an insulating material, as of hard rubber, as small particles from 
either surface are sure to be carried upon the other surface, thus 
forming a partial electrical connection on the insulating surface and 
a defective connection on the brass or metal surface. Where a long 
sliding contact is used much trouble is often caused by the cutting 



152 



AMERICAN TELEPHOXE PRACTICE. 



of the two surfaces. The extent of this cutting, even where the 
pressure is very light and the movement very limited, is often as- 
tonishing. 

In Fig. 133 is shown the hook-switch now almost universally 
used by the Bell Telephone Company, and known as the ''Warner 
Switch." The hook lever is pivoted to a bracket by a screw as 
shown, and is provided with a lug, f, and a strip of insulating 
material, g, on its short arm. On the under side of the lever is an 



3 Conductor 
Cord-sr 




FIG. 129.— BRIDGING DESK SET CIRCUIT— COIL ON RACK. 



insulating pin, h, and a contact point, i. A spring screwed to the 
generator box under the lever by the screw, b, bears alternately 
upon the insulating pin, h, and the contact point, i, and tends to 
press the lever into its elevated position. Springs, c and d, screwed 
to the side of the generator box, bear alternately upon the insulating 
piece, g, and the conducting lug, /. according to whether the lever 
is depressed or elevated. The spring, c, is connected through the 
secondary winding of the induction coil and the receiver to one side 
of the line. The screw, b, is connected through the calling ap- 



LOCAL BATTERY SUB-STATION EQUIPMENTS. 



153 



paratus to the same side of the line. The binding screw, a, con- 
nected with the lever, e, forms the terminal of the other side of the 
line. The local circuit terminates on one side of the spring, c, and 



\_ j AF'T" l T"" ...nHffr. 




FIG. 130.— GENERATOR CIRCUIT. 



on the other side in the spring, d. When the hook is depressed, 
point, i, is connected through the lower spring with the screw, b, and 
the calling circuit is complete. Both the local circuit and the line 




FIG. 131.— GENERATOR CIRCUIT. 



circuit through the talking apparatus are broken at springs, c and d. 
When the hook is elevated, the calling circuit is broken at the 
point, *, and the local and line circuits are completed by the springs, 
c and d, and lug, /. 




ENERATOR CIRCUIT. 



It cannot be denied that this piece of apparatus was for a long 
time the best switch-hook made, and that it gives satisfactory and 
reliable service now. It is, however, not self-contained as are most 



154 



AMERICAN TELEPHONE PRACTICE. 



modern hooks, its various springs and contacts being scattered 
around over the inside of the box, and therefore subject to faults 
due to inaccurate mounting, and to subsequent changes in position 
due to warping or shrinking of the woodwork. For the sake of 
economy, if for no other reason, it would seem that the Western 
Electric Company would adopt its customary modern methods in 
this case also. 

In Fig. 134 is shown the hook-switch now embodied in all the 
subscribers' instruments manufactured by the Kellogg Switchboard 
and Supply Company. The hook lever, L, is of brass, pivoted in a 
bracket, B, secured at the side of the box. It is normally main- 
tained in its raised position by a strong spring, S, secured to the 
bracket. Springs, 1, 2, 3, 4, and 5, are secured by two screws to 




(ren.y-fiinger* \ 



FIG. 133.— WARNER HOOK SWITCH. 



a lug on the bracket, B, these springs being insulated from each 
other by hard rubber blocks and bushings. Spring, 4, is made 
longer than the others, so as to engage in a slot in the lug, /, cast 
on the side of the hook lever. The springs are platinum-pointed, 
as shown, the arrangement being such that when the hook is in its 
raised position the spring, 4, presses 3, 2. and 1 together, leaving 
spring, 5, disconnected as shown. When, however, the hook is 
lowered by the weight of the receiver, the springs, 1, 2, and 3, 
break contact with each other and with spring, 4, the latter making 
contact with spring, 5. The long spring, 4, is connected with one 
side of the line, and serves to complete the talking circuits when the 
lever is raised, and the signaling circuit when the lever is depressed. 
This switch has a distinct advantage over most other types, in 
that the lever itself and the restoring spring, 5. form no part of the 



LOCAL BATTERY SUB-STATION EQUIPMENT. 



155 



circuit, and, therefore, no provision has been made to prevent loose 
contacts between them. The contact springs are all platinum- 
pointed, so that there is small liability of trouble. A strong point 
in favor of this form of switch is the ease with which it may be 




^r 



FIG. 134.— KELLOGG HOOK SWITCH. 



adapted to meet the requirements of almost any circuit, it being 
very easy to add more springs or to so arrange them that their 
contacts will be made and broken in a definite order upon the raising 
or lowering of the hook. 





FIG 135.-STROMBERG-CARLSON HOOK SWITCH. 



This method of mounting all of the springs of a group together, 

)cing fastened by screws and bushings in the manner shown', was 

or a long time typical of the Kellogg apparatus, but has now boon 

adopted by several other manufacturers. This hook, moreover, is 



156 



AMERICAN TELEPHONE PRACTICE. 



entirely self-contained, and is, in that respect, a decided improve- 
ment over many earlier types. 

Fig. 135 shows the hook-switch recently put on the market by 
the Stromberg-Carlson Company, which also has the advantage of 
being self-contained and of allowing a ready adaptation of the con- 




1 \ N Z 




FIG. 136.— MONARCH HOOK SWITCH. 

tact springs to any circuit requirements. In this the springs are 
mounted on a lug bent up from the base, the two outside springs 
being operated upon by a hard rubber cam carried on an upwardly 
projecting arm on the hook lever. This cam, when the hook is 
depressed, wedges the two long springs apart, thereby causing them 





FIGS. 137 AND 138.— FAULTY TYPES OF HOOK SWITCHES. 

to break contact with the two inner springs. This particular com- 
bination is adaptable to bridging telephones, the two long springs 
being wired together to form a single line contact, in which case 
the upper spring may carry the battery and the lower one the sec- 
ondary circuit. 

In Fig. 136 is shown another form of switch-hook recently put 



LOCAL BATTERY SUB-STATION EQUIPMENT. 157 

on the market by the Monarch Telephone Manufacturing Company. 
This hook also possesses a group of springs fastened together as in 
the two preceding types of hooks, and mounted upon a cast-iron 
base. The hook lever is pivoted to an upwardly extending lug on 
this base. The motion of the hook is imparted to the long spring 
by a vertical link member, as is clearly shown. The hook is raised 
when released from the weight of the receiver by a coiled spring 
within the base, this spring acting on a bell-crank arm extending 
downwardly within the base from the end of the lever. 

The lever of this hook is made in two pieces, being fastened to- 
gether by two screws, as shown. This enables the hook to be 
easily removed from the telephone without taking off the escutcheon 
plate used to cover the hole in the box through which the hook 
passes. 

Figs. 137 and 138 show two hook-switches which are examples 
of bad practice-. They have only the advantage of being self- 
contained, and cheap — no platinum being used. 



CHAPTER X. 

TELEPHONE LINES. 

Up to this point the apparatus used in transmitting speech and 
signals over telephone lines has been considered, but nothing has 
been said of the properties of the lines themselves. It is best to 
discuss these properties before passing to the subject of the tele- 
phone exchange, as the lines form an important part of the ex- 
change, the connecting links between the sub-stations and the 
central office. 

In the early days of telephony, the fact discovered by Steinheil, 
that the earth could be used instead of the return wire of an electric 
circuit, was made use of, and telephone lines were generally con- 
structed accordingly — that is, with but a single wire, using the 
earth as the return. Such lines are commonly termed ''grounded" or 
"ground return" lines, one of them being shown diagrammatically 
in Fig. 139, connecting two telephones. The ground connections 
in this figure are shown at G G, this being the now universally 
adopted way of showing such connections or "grounds" as they are 
called. 

Lines so constructed were soon found to be subject to serious 
difficulties, chief among which were the strange and unaccountable 
noises heard in the receiving instruments. There are many causes 
for such noises, some of which are not entirely understood. The 
swinging of the wire, in such manner as to cut through the lines 
of force of the earth's magnetic field, or the sudden shifting of the 
field itself, causes currents to flow in the line wire which may pro- 
duce sounds in the receiver. On long grounded lines the variation 
in the potential of the earth at the ground plates, due to any cause 
whatever, will cause currents to flow in the line. The passing of 
clouds or bodies of air charged with electricity will induce charges 
in the line, and cause currents to flow to or from the earth through 
the receiving instruments. Electric storms and auroral displays 
apparently greatly heighten these effects. These noises are of 
varying character, and Mr. J. J. Carty well describes them in saying: 

"Sometimes it sounded as though myriads of birds flew twittering 
by; again sounds like the rustling of leaves and the croaking of 

158 



TELEPHONE LINES. 



159 



frogs could plainly be heard; at other times the noises resembled 
the hissing of steam and the boiling of water." 

The noises due to these natural phenomena, whatever their true 
cause may be, are chiefly annoying on long lines, short lines being 
disturbed only during heavy electrical storms. This is not the case, 
however, with the noises arising from the proximity of other wires 
carrying varying currents. Telegraphic signals can be plainly 
heard in a telephone instrument on a line running parallel with a 
neighboring telegraph line for a very short distance. The estab- 
lishment of an electric railway or electric lighting plant in a town 
using grounded telephone lines will always cause serious noises in 
the telephones, and if the lighting current is alternating the use of 
the telephones is usually out of the question at night time, while the 
plant is running. 





FIG. 139.— GKOUNDED LINE. 



Disturbances on telephone lines from neighboring wires may be 
attributed to one or all of the following three causes: leakage, elec- 
tromagnetic induction, and electrostatic induction. 

Leakage may occur through defective insulation between the 
two circuits; or even when the insulation of the wires themselves is 
practically perfect a heavy return current from a grounded circuit, 
such as of an electric railway, may, upon its arrival at the grounded 
end of the telephone line, have the choice of two paths, one through 
the telephone line, and the other a continuation of its path through 
the ground. This is the greatest source of trouble due to railway 
work, on grounded telephone lines. 

Electromagnetic induction on a telephone line is due to the fact 
that the line wire lies in the field of force set up by current flowing 
in the disturbing wire. About every wire carrying a current there 
is a field of force, or "magnetic whirl," consisting of closed lines 



160 



AMERICAN TELEPHONE PRACTICE. 



of force surrounding the conductors. Such a condition is repre- 
sented in Fig. 140. If the current is a continuous one, the lines 
of force will not vary after being once set up, and the telephone 
wire lying in this field will not be affected. If the current in the 
disturbing wire is fluctuating, the number of lines of force in this 
field will vary; or, by a clearer way of expressing it, the field of 
force will expand and contract accordingly. This expansion and 
contraction of the field will cause its lines of force to cut the tele- 
phone wire, and will by the laws of electromagnetic induction cause 
currents to flow in the latter. If the current in the disturbing wire 
is an alternating one, the field of force around it will be established 
in one direction, destroyed and established in the reverse direction, 




FIG. 140.— MAGNETIC LINES AROUND A CONDUCTOR. 



and again destroyed, with every complete cycle of the current. 
This is the condition for a maximum disturbance in the telephone 



wire. 



Electrostatic induction may be explained by reference to Fig. 
141, where a grounded telephone line is shown running parallel 
with a disturbing wire, which we will say is carrying an alternating 
electric current. The disturbing wire will receive from its source 
of current alternate positive and negative charges of electricity, and 
its potential will pass from a maximum in one direction through 
zero to a maximum in the other, and again through zero to the 
maximum in the first direction during each cycle. 

Consider the condition when the potential of the disturbing wire 
is zero. No charge will then be induced on the telephone wire, so 



TELEPHONE LINES. 161 

that its potential will also be zero, unless subject to other influences. 
The charge on the disturbing wire then becomes, we will say, posi- 
tive, and this induces a bound negative charge on the side of the 
telephone wire nearest the disturbing wire, and an equal positive 
charge on the opposite side. This latter charge is not bound, and 
flows to earth through the receivers at each end. This flow will 
be toward the ground, through each receiver, and the current is 
therefore from the center of the wire in each direction to the ground. 
The next instant the potential of the disturbing wire becomes zero, 
thus relieving the bound negative charge on the telephone wire, 
which flows to earth, or, more properly, is neutralized by a flow of 
positive electricity from the earth. Thus each change in potential 
of the disturbing wire causes a flow of current through the receivers 
at each end, this flow always being toward or from the middle point 
in the length of the wire. These currents produce noises in the 
receivers at each end in the ordinary way. 
When two grounded telephone circuits run side by side, each 

PiSTUF\e>ir*5 wire: 

+- ± ±±±±±±±± ± + 



FIG. 141.— ELECTROSTATIC INDUCTION. 

acts inductively on the other, so that a conversation carried on over 
one circuit may be heard in the telephones on the other. This 
phenomenon is aptly termed cross-talk, and is usually explained in 
text-books and articles on the subject by the supposition that it is 
chiefly if not entirely due to electromagnetic induction. 

In 1889, however, Mr. J. J. Carty, in a paper before the New 
York Electric Club, and again in 1891, in another paper before the 
American Institute of Electrical Engineers,* described a series of 
experiments which show conclusively that cross-talk between lines 
is due almost entirely to electrostatic induction, electromagnetic in- 
duction playing so small a part as not to be noticeable. 

The arrangement of circuits in one of his experiments is shown 
in Fig. 142, in which E F and C D are two well-insulated lines, 
each 200 ft. long, and placed parallel with each other throughout 
their entire length, at a distance of J in. apart. E F is the disturb- 



* These papers are classics, and should be read by all interested in this subject. 
11 



162 



AMERICAN TELEPHONE PRACTICE. 



ing line and is left open at E. At F it is connected through the 
secondary of an induction coil, L, with the ground. In the primary 
circuit of this coil is a battery, B, and a Blake transmitter, T. A 
tuning fork vibrating before the transmitter acted on the diaphragm 
in the usual way, and caused impulses on the line E F of practically 
the same strength as voice currents. These impulses are, of course, 
alternately positive and negative, and may be considered in the 
same light as the impulses on the disturbing line in Fig. 141. Three 
receivers, x, y, and z, were placed in the line CD, the receiver, y, 
being at the middle point in the line. Upon operating the tuning 
fork, its musical note could be distinctly heard in receivers, x and z, 
while y remained silent. 

In explaining the action of static induction in connection with 
Fig. 141, it was pointed out that the flow of induced currents would 
be either toward or from the middle point in the length of the wire. 



i 



— B 

, I— vwyy 



c -r- 



(i 



h- -t- -h 



vww 



r d 



FIG. 142.-ELECTROSTATIC INDUCTION. 



The silence of the receiver, y, in this case bears out that statement, 
showing the central point to be neutral. If this were electromag- 
netic induction, the induced current would pass from one end of 
the line, C D y to the other, returning through the ground, in which 
•case all the receivers would be affected. As it is, however, the in- 
duced charges flow in each direction from the receiver, y, to the 
ground at each end, or from the ground at each end to the receiver, 
y, thus in no case causing *ts diaphragm to vibrate. The same 
results were obtained by grounding the point E through an or- 
dinary telephone. The receiver, y, still remained silent, while x 
and z were both affected to an equal degree. 

It was also found that opening the central point of the line, C D, 
produced no effect whatever on the existing conditions; the noises 
in the receivers, x and z, were plainly heard and of equal loundess. 

Many other experiments were tried, the results in each case 



TELEPHONE LINES. 



163 



pointing conclusively to the induction from voice currents being of 
an electrostatic instead of an electromagnetic nature. 

There is no doubt, however, that induction from wires carrying 
heavy currents, such as are used in lighting and power work, is 
partly due to electromagnetic effects, and this can be easily proven 
by experiments similar in nature to those described. 

The one remedy for all the troubles due to disturbing noises from 




FIG. 143.— METALLIC CIRCUIT LINE. 

any of the causes is to make the line a complete metallic circuit. 
By this is meant a circuit having both its sides formed of a wire, 
each wire being individual to the circuit and not forming a portion 
of other circuits. A metallic circuit line connecting two telephones 
is shown in Fig. 143. 

This alone, however, will not completely stop noises from most 
of the causes, and additional precaution must be taken, by making 
the two sides of the circuit alike in all respects and properly trans- 
posing them at frequent intervals, in order that they may be as 



DISTURBING WIRE 



P=^c 



3>o=^) 



FIG. 144.— ELECTROMAGNETIC DISTURBANCES. 



nearly symmetrical with respect to the disturbing source or sources 
as possible. 

Merely making the line a metallic circuit, as in Fig. 143, does 
not give complete freedom from inductive troubles from other wires, 
whether the induction be electromagnetic or electrostatic. Con- 
sidering the question from the standpoint of electromagnetic induc- 
tion, a current flowing in the disturbing wire, Fig. 144. would set 
up a field of force, the lines of which would cut conductors. A and 



164 AMERICAN TELEPHONE PRACTICE. 

B. A being closer, however, would be cut by more lines than B y 
and consequently any currents induced in A by changes in this field 
will be stronger than those in B. If a current starts to flow in the 
disturbing wire from right to left, as shown, the induced currents 
in A and B will each be from left to right, as indicated by the arrows. 
These currents will partially annul each other, but that in A, being 
the stronger, will predominate, and the resultant will flow in the 
circuit in a direction indicated by the small curved arrows. 

A single transposition in the center of the metallic circuit will 
completely annul the electromagnetic induction if the disturbing 
wire is parallel to the two wires throughout its entire length, and 
if it carries the same current in all its portions. Here an impulse 
in the direction of the arrow in the disturbing wire (Fig. 145) will 
cause impulses in the opposite direction in both wires, A and B. 
As the average distances between the disturbing wire and A and 
B, respectively, are the same, the strength of the induced currents 



DISTURBING WIRE 



FIG. 145.— ELECTROMAGNETIC DISTURBANCES. 

in A and B will be equal, and they will, therefore, annul each other, 
producing no sound in the receivers. 

It is found, however, that a single transposition in the center of 
the metallic circuit will not free the line from cross-talk, even 
though the average distance from the two wires and the disturbing 
wire is the same, and the current strength is uniform throughout 
the entire length of the disturbing wire. 

Mr. Carty's experiments throw much light on this point. In 
Fig. 146 is shown a disturbing wire and a metallic telephone cir- 
cuit composed of two wires, A and B, of which A is nearer the 
disturbing wire than B. At a time when the charge on the disturb- 
ing wire is positive, as shown, a negative charge will be drawn by 
it toward the disturbing wire and a positive charge will be repelled 
from it. The result is that the distribution of charges on the two 
wires, A and B, will be somewhat as shown, a negative charge being 
held on the wire, A, and a positive charge driven to the wire, B. 

In order for this rearrangement to have occurred, it is evident 
that a flow of electricity must have taken place from A to B, and 
as two paths were afforded from the center point, a, on the wire A> 



TELEPHONE LINES. 165 

of equal resistance, this flow must have been from that point in 
each direction as indicated by the arrows, through the receivers and 
toward the center point, b, on wire, B, where the two currents met. 
Upon the charge on the disturbing wire becoming zero the poten- 
tials on A and B become equal, by a flow of positive electricity from 
the center point of wire, B, to that of wire, A. The negative charge 

DISTURBING WIRE. 

-t--l--l-4-+ + -t- + 4- 

B b " 

FIG. 146.— ELECTROSTATIC DISTURBANCES. 

on the disturbing wire, which follows the positive charge, will cause 
this latter to flow from b to a, until A is positively and B negatively 
charged. 

It is evident, therefore, that alternating currents flow through the 
two receivers, and that these currents differ in phase from that in 
the disturbing wire by 90 degrees, which is characteristic of the 
action of condensers. Further consideration will show that the 
points a and b are neutral, and experiment bears out this conclusion, 
for by opening the wires at those points the sound in the receivers 
at the ends still continues. Again, if receivers are connected in the 
circuit at a and b no sound is heard in them, although plainly audible 
in the end receivers. 

A single transposition in the center of the line, as shown in Fig. 
147, will tend to reduce the sound in the end receivers, but will not 

DISTURBING WIRE 
+ + + + + + + + + + 



P*+ ± 4- ±y X-t- ± + ±zj 

T & A' *f 

FIG. 147.— ELECTROSTATIC DISTURBANCES. 

cause silence. The static charges on the portions of the wires 
nearest to the disturbing wire now find four paths instead of two 
to the more remote portions of the circuit, the flow being clearly 
indicated by the arrows. The center points, a and b, are no longer 
neutral, and receivers placed in the circuit there will be subject to 
noises. 



166 



AMERICAN TELEPHONE PRACTICE. 



It is evident that if receivers of equal impedance to those at the 
ends of the line were placed at a and b, the neutral points, c, d, e, 
and f t would be found at the quarter points on the line; i. e., midway 
between the transposition and each end. As a matter of fact, how- 
ever, no instruments are placed at the point of transposition, and 
the neutral points are shifted toward the ends of the line, because the 
impedance of the receivers at those points makes it easier for most 
of the current to pass through the transposition wires. 

Theoretically, the currents set up in a metallic circuit by electro- 
static induction from another circuit can be eliminated only by 
making an infinite number of transpositions. Practically, however, 
it is found that on long circuits transpositions every quarter or 
half-mile are sufficient to render them unnoticeable. 

The scheme of transposition used by the American Telegraph 



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FIG. 148.— DIAGRAM OF TRANSPOSITIONS. 



and Telephone Company on the New York-Chicago telephone line 
is shown in Fig. 148. It will be seen from this figure that trans- 
positions are made on this line practically four times in every mile, 
that is, upon every tenth pole. While this involves the placing of 
transposition insulators on poles a quarter of a mile apart, it does 
not follow that every circuit is transposed at each of these intervals. 
The reason tor this arrangement is that if two lines running side by 
side were transposed in exactly the same manner throughout their 
lengths, the desired non-inductive condition would not be secured, 
for the relation between the corresponding wires in the two circuits 
would then be the same as if no transposition whatever had been 
made. In order to overcome this difficulty, transpositions on the 
second circuit should be made twice as often as those on the first. 



TELEPHONE LINES. 167 

This is the scheme adopted in Fig. 148, where it will be seen that 
the center pair of wires on each set of cross-arms is transposed every 
mile, while the pair immediately adjacent to it on each side is trans- 
posed twice as often. The outside pairs on each cross-arm are 
transposed only once in each mile, but these transpositions are 
staggered with respect to those on the • center pair. The same 
scheme is followed out on every cross-arm, but the transpositions 
on the top set of cross-arms are staggered with respect to those on 
the set immediately below — this being the case throughout the 
entire number of cross-arms on a pole; the 1st, 3d, 5th, 7th and 
9th being transposed according to the scheme shown in the upper 
part of Fig. 148, while the circuits on arms Nos. 2, 4, 6, 8 and 
10 are transposed according to the scheme in the lower part of this 
figure. 

A very perfect transposition is effected by twisting two sides of 
a circuit together, and this idea has been followed to some extent 



ST/VTIOI 



REPEATING COIL. 



METALLIC UMfe I Sv> I gROUNPtP LIME. 




3rTAn6N 

7® 



FIG. 149.— CONNECTION OF METALLIC TO GROUNDED LINES. 

in European pole-line construction, where the two sides of the cir- 
cuit are not only transposed laterally, but also pass successively 
over and under each other several times in each mile, thus effectu- 
ally giving the circuit a number of complete twists. This method, 
however, involves several disadvantages in the stringing of wires, 
and increases the liability of crosses between them. 

The twisted pair of insulated wires used so largely in inside 
wiring, and also in cable work, accomplishes the transposition of 
circuits very thoroughly, it in fact amounting to a complete trans- 
position for every twist of the wires. This method is now depended 
upon entirely in the construction of telephone cables, with so great 
a degree of success as to absolutely prevent all induction between 
the circuits. This will be discussed at greater length in the chap- 
ter on cables. 

It frequently becomes desirable to connect a grounded line with 
a metallic line, and for this purpose what is known as the repeating 



168 AMERICAN TELEPHONE PRACTICE. 

coil forms the most ready solution. A repeating coil is merely a 
special form of induction coil, having two windings on a single core, 
these windings often being the same in number of turns and re- 
sistance. They are frequently completely enclosed in iron, for the 
purpose of affording a more complete magnetic circuit for the 
magnetic lines of force set up by currents in the coils. In Fig. 149 
is shown in diagram a metallic circuit line connected with a 
grounded line through a repeating coil. The two terminals of the 
metallic circuit line are merely brought to the two terminal binding 
posts of one of the coils, while the end of the grounded line is 
brought to one of the terminal posts of the other coil; the remaining 
post is then grounded. Any varying currents set up in one of the 
circuits will act inductively on the other circuit through the wind- 
ings of the coil, each of which may thus be called upon to act alter- 
nately as a primary and a secondary. By the use of the repeating 
coil in this manner, two lines may be connected for conversation 



DISTURBING WIRE! 




FIG. 150.— ELIMINATING LOCAL INDUCTION. 

without grounding one side of the metallic circuit, which would be 
necessary were the repeating coil not used. 

There is a very common impression among many telephone users 
that a repeating coil is a panacea for all of the evils connected with 
grounded lines. It is perhaps well to correct this impression by 
saying that no number of repeating coils will render a noisy 
grounded line quiet. A repeating coil will, however, prevent the 
unbalancing of a metallic circuit line, and therefore in many cases 
insure a degree of quietness on two connected lines which would 
otherwise be unattainable. 

It sometimes happens that a long grounded line is paralleled 
throughout only a portion of its length by some disturbing wire, 
such, for instance, as an electric light line. Where it is not possible 
from commercial considerations to make the entire line a metallic 
circuit, relief may sometimes be had by resorting to the plan shown 



TELEPHONE LINES. 169 

in Fig. 150, which consist in making only that portion of the line 
a metallic circuit which is within the direct influence of the disturb- 
ing wire. The two ends of the grounded line may then be con- 
nected with the intermediate metallic portion by means of the re- 
peating coils, RR, as shown. By this arrangement the disturbing 
wire produces no effect on the metallic circuit between the repeating 
coils, if proper precautions are taken in the way of transposing its 
two sides. Telephonic communication may be had over the entire 
length of line, the currents undergoing two transformations at the 
repeating coils. 

Much trouble is often had where it is necessary to ring through 
repeating coils, especially if the lines are very long. It is therefore 
advisable that repeating coils should always be placed, if possible, 
at a station where there is an attendant, and such arrangements 
made that it will not be necessary to ring through them. However, 
a coil properly constructed with a magnetic circuit completely 
closed should serve as a fairly efficient transformer, even for the 
slowly alternating currents of a magneto generator, and good re- 
sults may be obtained with such coils on good lines even when it 
becomes necessary to ring through them. 



CHAPTER XI. 

THE TELEPHONE EXCHANGE IN GENERAL. 

The object of a telephone exchange is to afford means for tele- 
phonic intercommunication to a community. The word com- 
munity may be restricted to a small body of people having common 
interests, as, for instance, those within the employ of some firm, 
perhaps housed within a single building; it may refer to the people 
of a city with its suburbs; to the people of an entire section of a 
country, or, in its largest sense, to the people of a whole country 
or perhaps of a continent enjoying the same laws, rights and privi- 
leges. Upon the scope given the word community, therefore, de- 
pends our idea of a telephone exchange. 

It is unquestionably proper to refer to an organization embodying 
but a few telephones placed within the offices of a business firm for 
supplying telephone service to its various employees as a telephone 
exchange. Again, it is proper to refer to the telephonic organiza- 
tion of a great city with its outlying suburbs, as a telephone 
exchange. These two ideas of the exchange are well accepted. 

If, however, we use the term community in its broadest sense as 
applying to the people of an entire country or continent, it is also 
thought proper to refer to the telephone system of that country as 
an exchange, wherein the systems of the large cities are connected 
by long-distance lines with each other, the small cities all being 
connected by similar lines to their nearest larger city, thus affording 
means for the people in any portion of this great community to se- 
cure telephone communication with those in any other portion. This 
idea is in accordance with the following statement, made in a lec- 
ture by the writer several years ago : 

"Those who have the best interest of the telephone business at 
heart are now designing their apparatus and circuits on the basis 
that telephone systems in small villages must have as good trans- 
mission as those in the largest cities, with the idea in view that in 
the future the whole continent will be one vast telephone exchange, 
the various large cities being the main offices, and the long-distance 
lines, the trunk lines between them, and the small villages the branch 
exchanges." 

170 



TELEPHONE EXCHANGE IN GENERAL. 171 

It is usual to have the subscribers in a certain community, or 
where this is too large, in a certain portion of the community, con- 
nected by means of telephone lines with a certain central point, at 
which apparatus is provided for inter-connecting lines in accordance 
with the wishes of the subscribers. Such a central establishment is 
usually called a central office, or a telephone office. May we not then 
define as follows : 

A telephone office is an establishment in which telephone lines 
center, containing equipment for interconnecting the lines. 

A telephone exchange is an organization of one or more telephone 
offices and the connecting lines and sub-station equipments necessary 
for supplying telephone service to a community. 

Under these definitions a telephone exchange may consist of one 
or more telephone offices. A telephone office may in itself form the 
only means of connecting the subscribers in an entire exchange, or 




o 

FIG. 151.— SINGLE-OFFICE EXCHANGE. 

it may form the center of the lines of only one group of subscribers 
in an exchange. 

Where there are more offices than one in an exchange, lines must 
be provided extending between them to afford a means for connect- 
ing a subscriber's line terminating at one office with the line of a 
subscriber terminating at another office. 

The simplest form of an exchange is that containing but one cen- 
tral office, this office being isolated so that no means is provided 
for connecting it with other offices. Such an exchange may be repre- 
sented diagrammatically as in Fig. 151, wherein a large circle in the 
center represents the central office, the small circle surrounding it 
represents the subscribers' stations or sub-stations, as they are more 
commonly called, and the lines connecting the large circle with the 
respective small circles, the subscribers' lines. 

In Fig. 152 is shown an exchange having two central offices, each 



172 



AMERICAN TELEPHONE PRACTICE. 



represented in a manner similar to that shown in Fig. 151, and hav- 
ing extending between them trunk lines through the medium of 
which any subscriber's line in one office may be connected with any 
subscriber's line in the other. 

In Fig. 153 is shown an exchange having three central offices, each 
office being connected with each of the other two by means of trunk 
lines. In a similar manner we might indicate by a diagram an ex- 
change for a very large city having, perhaps, twenty-five telephone 
offices, and in this case a trunk line should be extended from each 
office to each one of the twenty-four others. 

Exchanges may be classified in several ways : Thus, as single 
office exchanges or multi-office exchanges, according to whether 
there are one or many offices. 

If classified according to the kind of lines used, we may have 





FIG. 152.— TWO-OFFICE EXCHANGE. 



metallic circuit exchanges, grounded line exchanges, or common 
return exchanges. 

If classified on still another basis, we may have manual exchanges, 
where the connections at the central offices are made manually by 
means of operators, in accordance with the spoken desires of the 
subscribers ; or we may have automatic exchanges, where automatic 
machines are provided at the central offices manipulated over the 
lines by the calling subscribers to complete the connection as de- 
sired. 

Again, we may classify according to the location of the sources of 
electrical energy for supplying current to the subscribers for talking 
and signaling. Under this classification we have a local battery 
exchange, where each sub-station apparatus contains a local bat- 
tery for supplying current to his transmitter, or we may have a com- 
mon battery or central energy exchange where all sources of energy 
for both calling and talking are located at the central office. In the 
local battery exchange the sub-station apparatus usually comprises 



TELEPHONE EXCHANGE IN GENERAL. 



173 



a magneto generator for enabling the subscriber to call the central 
office, for which reason such an exchange is often spoken of as a 
magneto exchange. 

It may be said that the grounded circuit or common return ex- 
changes are gradually giving way to the exchange using only com- 
plete metallic circuit lines, and that the local battery or magneto 
exchanges are gradually giving way to the common battery or cen- 
tral energy exchange wherein all sources of electrical energy are 
located at the central office. Particularly is this true in large ex- 
changes. The magneto or local battery exchange still has, however, 




FIG. 153.— THREE-OFFICE EXCHANGE. 



a wide field of usefulness in small exchanges, and will probably never 
be completely supplanted. 

In an automatic exchange wherein no operators are supposed to 
be employed, each subscriber must have at his own station, mechan- 
ism which, when properly operated by him, will continue the circuit 
of his line through auxiliary circuits and mechanisms at the central 
office, to the line of the subscriber desired, and after this connection 
is made to ring the bell of that subscriber. Further means must be 
provided by which the subscriber may clear his line from such con- 
nections after he is through talking. 



174 AMERICAN TELEPHONE PRACTICE. 

It may be said that up to the present time the automatic exchange 
has come into only very limited use, although there is at present a 
movement on foot which promises to have installed automatic ex- 
changes in several large cities of this country within a comparatively 
short time. The success of this movement will be watched with 
great interest. 

In every manual exchange means must be provided for enabling 
the subscriber desiring a call to attract the attention of the operator 
at the central office to which his line extends. Means must also be 
given the operator for connecting her telephone to the subscriber's 
line in response to a call in order that she may ascertain his wishes. 
The operator must also be provided with facilities for ringing the 
bell of the subscriber called for, in order to attract his attention, 
and thereafter, for connecting the lines of the two subscribers for 
conversation. Lastly, the subscribers must be afforded means for 
attracting the attention of the operator after the connection has been 
made, in order to inform her that the connection is no longer desired, 
or that another one is wanted. 

The first idea of central office exchange work that I am able to 
find suggested, is in a British patent to Dumont, in February, 185 1. 
This imperfectly describes a system of telegraph lines radiating from 
a central station, with means at the central station for placing any 
line in telegraphic connection with any other. Like the idea of 
Bourseul, this of Dumont was long ahead of its time — occurrences 
found in the early development of almost every industry. 

In 1874, before the birth of the telephone, a telegraph exchange 
system was put into use in New York City. It was for connecting 
lawyers who chose to subscribe, and was, for this reason, called the 
law system. The telegraph instruments, which were of the dial pat- 
tern, were subsequently replaced by telephone instruments, and thus 
originated, as far as I am able to find out, the first telephone ex- 
change system. It was from this particular system that the well- 
known Law telephone system, widely used in later years, took its 
name. 

All of the early telephone switch-boards by which the lines were 
connected, as described by the subscribers served, were built upon a 
plan which had proven successful in telegraphy; the lines usually 
terminating in vertical conducting strips across which were placed 
horizontal conducting strips for connectors. Any two lines could be 
connected together by connecting each by means of a plug to the 
same horizontal strip. Indicators in the form of electromagnetic 



TELEPHONE EXCHANGE IN GENERAL. 175 

drops, or annunciators, were used in connection with each line, and 
also spring switches, or spring jacks, by means of which the opera- 
tor could connect her telephene in the circuit with any line. 

It very soon became apparent that a far greater number of lines 
would have to be handled in the telephone business than in teleg- 
raphy, and for this reason the old telegraph switchboard soon be- 
came inadequate. In order to met the new demands, the develop- 
ment began in a new direction. Each line terminated in a spring 
jack, and an indicator, and a connection was made between any two 
lines by means of flexible conducting cords terminating in plugs 
adapted to fit into and make connection with the spring jacks. Thus 
we find about the beginning of the eighties, the prototype of our 
modern switchboard. The development of the switchboard will be 
treated in many of the subsequent chapters in this book. 



CHAPTER XII. 



THE MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 



By a magneto switch-board is meant a switch-board adapted to 
interconnect lines, equipped with magneto sub-station apparatus. 
In other words it is a switchboard, the signal-receiving apparatus 
of which is adapted to be operated by magneto generators at the 
subscribers' stations. 

For the sake of simplicity a switch-board for grounded cricuit lines 
will be first discussed. In Fig. 154 the numerals 1 to 12 inclosed in 




FIG. 151— GROUXDED-CIRCUIT SWITCH-BOARD. 

circles represent sub-station apparatus, it being understood that each 
such apparatus comprises the usual call sending and call receiv- 
ing apparatus of the magneto type, as well as the usual talking ap- 
paratus. One terminal of each sub-station apparatus is connected 
with the earth, as indicated, the other being connected to the line 
wire leading to the central office. At the central office each line wire 
passes first through a switch socket or spring- jack, represented by 
a small circle, and then through the winding of the annunciator or 
electro-magnetic drop, represented by a larger circle. From this the 

176 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 177 

circuit of each line passes to a common wire leading to the earth. 
The spring- jack serves the purpose of allowing the operator to make 
a connection between a flexible conductor and any line circuit, while 
the drop serves the purpose when actuated by a current from the 
subscriber's generator to display a visual signal to the operator, as 
an indication that her attention is required 1 on that line. After the 
call has been received on a line, the drop is of no further use, while 
the line is being used by the subscriber for conversation, and there- 
fore arrangements are made in each spring-jack to cut off the drop 
of each line when the connection is made. Each operator is provided 
with a number of pairs, c, of flexible conducting cords and plugs by 
means of which and the spring- jacks she may connect any two lines 




FIG. 155.— GROUNDED-CIRCUIT SWITCH-BOARD. 

for conversation, at the same time cutting off the call-receiving de- 
vices or drops belonging to those lines. The operator is also provided 
with a telephone set represented at T, and a calling generator repre- 
sented at G. One terminal of each of these is connected with the 
ground 1 , as shown, and the other terminal of each may be put in 
connection with a flexible cord or plug, enabling the operator either 
to converse with w a subscriber by means of her telephone set or 
to ring the bell of a subscriber who is called for, by means of her 
generator. 

Looking now at Fig. 155, we see that lines 2 and 3 have been con- 
nected for conversation by means of a flexible cord, 0, similarly lines 
7 and 8 and lines 4 and 11 have been connected by cords b and c. 
12 



178 



AMERICAN TELEPHONE PRACTICE. 



Each of these pairs of lines., it will be seen, are cut off from the call- 
receiving devices, and each pair of subscribers thus enjoys an ex- 
clusive circuit extending between their respective stations. Line No. 
6 is shown in Fig. 155 as connected with the operator's telephone, T, 
the operator having connected her telephone with the line by means 
of a jack and the flexible cord, in response to the call from that sub- 
scriber. Line No. 12 is shown in the conditions that would exist 
when the operator was calling the subscriber at its sub-station, the 
generator, G, having been connected by the operator with the spring- 
jack of that line, thus also cutting off the annunciator or drop. 

It will be seen from this very elementary discussion that the cir- 
cuit of each line is normally completed to earth at the central sta^ 
tion through the spring- jack and the annunciator. By means of a 
plug and flexible cord the operator may connect her telephone with 




FIG. 156.— SPRING-JACK. 



the line, thus changing the circuit of the line to include the operator's 
telephone instead of the drop. Also by means of a plug and flexible 
cord, the operator may connect the calling generator with a line, 
thus changing the circuit to include the generator instead of the 
drop. Again, by means of two flexible cords and plugs the operator 
may connect any two lines for conversation, cutting out the annun- 
ciators of both lines. Upon the withdrawal of any plug the line is 
at once connected again with its annunciator. 

In Fig. 156 is shown a simple spring-jack with the connecting 
plug inserted. The metallic base, a, of the jack, usually of brass, is 
drilled from its forward end to receive the shank of the plug,P. 

A forwardly projecting sleeve on this base fits snugly into a 
hole bored in the front board, A, of the switch-board, to which 
it is fastened by the shoulder and small wood-screw, as shown. 
Firmly secured to the rear end of the piece, a, is the line spring, 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 179 

c, formed with a rearwardly projecting tongue, to which the wire, /, 
leading from the line is soldered. The forward end of the spring, c, 
rests normally against the pin, p, carried by, but insulated from, 
the base, a. A wire, g, leads from this pin, and through the coil 
of the line annunciator or drop to ground. When the plug is in- 
serted in the jack its conducting tip makes contact with the tip of the 
line spring and at the same time forces it out of engagement with 
the pin,./>. Normally, therefore, the line wire is connected to the 
ground through the wire, /, spring, c, pin, p, wire, g, and line-drop 
to the ground connection. When the plug is inserted in the jack, 
however, the line is disconnected from the branch leading through 
the drop, but is connected through the medium of the plug to the 
flexible cord. 

Fig. 157 shows a common form of switch-board drop. The pur- 




FIG. 157.— SWITCH- BOARD DROP. 



pose of the drop is to attract the attention of the operator whenever 
any subscriber wishes a connection. The coil of the electromagnet 
is mounted on the back of the front plate, c, of the switch-board, as 
shown. To the armature, a, pivoted at its upper end, is attached a 
rod, b, passing forward through a hole in the front plate and pro- 
vided with a hook on its forward end, adapted to engage the upper 
portion of a pivoted drop-shutter, s, and to hold it in its raised posi- 
tion. The attraction of the armature due to a current passing 
through the coil causes the hook to rise, thus releasing the shutter, 
which falls to a horizontal position and displays to the operator the 
number by which that line is designated. 

In order to attract the attention of the operator at night or at 
such times as she may not be in sight of the board, a night-alarm 
attachment is provided on each drop, which serves to close the cir- 



180 



AMERICAN TELEPHONE PRACTICE. 



cuit through a battery and vibrating bell whenever the shutter is 
down. The small cam surface on the lower portion of the shutter, s, 
forces the light spring, t, into contact with the pin, t', when the 
shutter is down, thus accomplishing the above result. 
Fig. 158 shows diagrammatically the circuits of two lines con- 



Can 



/r'w 



Am 






?w 5 

FIG. 158.— CIRCUITS MAGNETO SWITCH-BOARD— NORMAL. 

nected at the central office to ground through their spring-jacks, 
/, and annunciators, D. Between these lines is also shown the cir- 
cuits and apparatus of a pair of plugs, P and P 1 with their flexible 
cords c and c\ together with the operator's telephone set, 7, the 
magneto generator, G, and the keys, K 1 , by which either the talking 
apparatus or the calling generator may be put in circuit with the 
plugs. 

Considering now the means by which a subscriber may attract the 
attention of the operator at the central office, reference is made to 
Fig. 159, which, it will be seen, shows the complete apparatus of 
one line only, as indicated in Fig. 158. By turning the crank of his 



tine 





FIG. 159.— CIRCUITS MAGNETO SWITCH-BOARD— SUBSCRIBER CALLING. 

generator without raising the receiver from its hook a subscriber 
at this sub-station may actuate the drop, D, at the central office over 
the circuit extending from ground at his station through his gener- 
ator and bell to line, thence through the spring-jack, /, and its anvil, 
a, to earth at the central office through the drop, D. This current 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 181 

will cause the release of the shutter, attracting the attention of the 
operator. Seeing the signal she will at once respond by placing 
the answering plug, P, of the pair into the jack, the spring of this 
jack making contact with the tip of the plug, and at the same time 
being raised clear of the anvil, a, thus cutting the drop out of cir- 
cuit. By depressing key, K, the operator is enabled to converse 
with a subscriber who has, meanwhile, removed his receiver from 
its hook, thus placing his talking apparatus in the circuit of the 
line, rendering his generator and call bell inoperative. The circuit 
conditions are now those shown in Fig. 160. The operator's talk- 
ing apparatus, T, at the central office consists of a transmitter and 
battery placed in the local circuit containing the primary winding of 
the induction coil, /. The secondary of this induction coil is in- 
cluded in the operator's receiver in the circuit between the earth 
at the central office and the anvil of the switch, K. The circuit 



ef -fine 




FIG. 160.— CIRCUITS MAGNETO SWITCH-BOARD-OPERATOR ANSWERING. 

over which the subscriber is able to converse with the operator is 
obvious. Having learned the number the subscriber wanted, the 
operator inserts the calling plug, P 1 , associated with the plug, P, 
into the jack of the calling subscriber, and depresses the ringing key, 
K 1 . The current sent out from the generator traverses the line of 
the calling subscriber and passes to earth through his bell. The 
parts of the circuit operative at this time are shown in full lines in 
Fig. 161. In response to this call, the subscriber raises his receiver 
from its hook, thus placing his talking apparatus in condition for 
use. Meanwhile the operator, having released the key, K l , the 
current of the calling generator is cut off from the line, and the 
talking circuit is made complete. This circuit over which the con- 
versation takes place is shown in Fig. 162. 

In Figs. 159 to 162, inclusive, those parts actually engaged in the 
particular operation illustrated by each figure, are shown in heavy 



1S2 



AMERICAN TELEPHONE PRACTICE. 



lines, the parts which, for the time being, do not enter into the 
operation, being shown in dotted lines. 

Since during conversation both line drops are cut out of circuit, 
it becomes necessary to provide means for enabling either or both 



#^i 



Zifre _ c/ 




j cf tine 



i W%. 



L 



X 




FIG. 161.— CIRCUITS MAGNETO SWITCH-BOARD— CALLING SUBSCRIBER. 

subscribers to again attract the attention of the operator, in order 
to signal that the conversation is finished, or for the purpose of 
asking for another connection. For this purpose, the clearing out 
drop, C 0, is provided in connection with each cord circuit, this drop 
being bridged between the cord and ground at the central office, and 
therefore ready to respond to the calling current sent out from either 
subscriber's generator. At the end of the conversation either sub- 
scriber, by turning his generator crank, after having hung up his 
receiver, is enabled to send a current over the circuit of the line, 
which passes to earth through the clearing-out drop at the central 
office as a signal to the operator. In these figures but a single pair 
of cords and plugs with their corresponding keys and clearing-out 
drop, are shown, in order not to confuse the diagrams. It is usual, 









4fr 

FIG. 162.— CIRCUITS MAGNETO SWITCH-BOARD— SUBSCRIBERS 
CONVERSING. 

however, to place approximately io such pairs for each ioo sub- 
scribers in the system, it being found that this number is sufficient 
to meet the requirements at the busiest periods of the day. This, 
of course, provides service permitting one of io per cent, of the sub- 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 



183 



scribers to be engaged in conversation with another 10 per cent, of 
the subscribers at the same time. But a single operator's telephone 
set is provided for each operator as, by means of the keys, K, asso- 
ciated with the various cord circuits she may, at will, connect her 
telephone set with any one of the pairs of cords. Likewise, a single 
generator, G, may serve the entire exchange. 

The line drops in a board of this type are wound to a resistance 
of about 80 ohms, unless designed for multiple or bridged telephone 
lines, in which case the resistance of the drops is usually made much 
higher, from 500 to 1000 ohms. The clearing out drops are usually 
wound to a resistance of 500 ohms. 

It has already been pointed out that in order to avoid induction 
and other sources of trouble, metallic circuits are rapidly super- 
seding ground circuits in telephone exchanges. The switch-boards 
in common use for small metallic-circuit exchanges are built on the 
same general principles as those for grounded circuits just de- 
scribed, differing from them only in such details as to render 



sLine \ 


c^ 


BSs 


Im / 


~~~~~~^=— 1 




,eoil ^j 


F?— \ 


» vOjpyifliilil 


""TVtlW-i 






§ } 




pi* 1 








— =1 







FIG. 163.— METALLIC CIRCUIT JACK. 



possible the connections of the two sides of one line with those of 
another line through the cord circuits. For this purpose two sepa- 
rate contacts are provided in each jack forming the terminals of the 
two sides of the line. The plugs also have two separate contact- 
pieces adapted to register with the contact-pieces in the jack when 
a connection is made. Each contact on one plug is connected to 
a similar contact on the other plug of a pair through the medium of 
a double-conductor flexible cord. 

One form of metallic circuit jack is shown in Fig. 163. Here 
the tubular portion, a b, forms a terminal for one side of the line, 
while the flexible spring, d, forms the terminal for the other side. 
The terminal, g, connected with the pin upon which the spring. d, 
normally rests, forms one terminal for the coil of the line-drop. The 
other terminal of this coil is attached to the terminal, a, so that when 
the spring, d, is in contact with its pin the circuit is complete from 
one side of the line to the other through the drop coil. The tubulai 



184 AMERICAN TELEPHONE PRACTICE. 

frame of this jack is made in two pieces, a and b. The front portion, 
b, is a hollow screw, threaded to engage a tapped hole in the front of 
the piece, a. By this arrangement any jack may be readily re- 
moved from the board by unscrewing the piece, b, until it disen- 
gages the rear portion, a. A slot for receiving a screw-driver is pro- 
vided on the front of the piece, b, to accomplish this. 

In Fig. 164 is shown another form of metallic circuit jack. This 



FIG. 164.— METALLIC CIRCUIT JACK. 

jack is self-contained and is mounted on the board by means of a 
screw-threaded thimble, in much the same manner as the jack shown 
in Fig. 163. The two springs are secured rigidly to the frame of the 
jack, but are insulated from it and from each other by strips of hard 
rubber and by insulating bushings for the screws. This is typical 
of good modern construction where the jacks are individually 
mounted. 

A metallic-circuit plug in common use is shown in Fig. 165. The 
tip conductor is formed of a rod of brass slightly enlarged at its 
forward end. This is encased in a bushing, b, of hard rubber, and 
over this is slid a tube, s, of brass forming the sleeve of the plug. A 
second bushing, b 1 , covers the rear portion of the sleeve, s, and the 
rear portion of this latter tube is in turn covered by the tube, b" , of 
hard rubber, forming the handle of the plug. The tube, s, forming 
the sleeve, has a portion which projects rearwardly into the handle, 
and is there provided with a connector, c, to which the terminal of 
one conductor of the flexible cord is attached. The other con- 




FIG. 165.— METALLIC CIRCUIT PLUG. 

nector, c', is attached to the rear portion of the tip piece, t, ana forms 
the terminal for the other conductor of the cord. 

In Fig. 166 is shown another two-conductor plug with cord 
attached, the plug being cut partly away so as to give a better under- 
standing of its internal construction. The cord is also partially 
dismembered for the same purpose. It will be seen that the cord 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 185 

has two conductors, each composed of twisted tinsel with a few 
strands of copper wire to give additional strength and conductivity. 
Around each conductor separately is placed a wrapping of silk and 
then a braiding of cotton, after which the two conductors, together 
with a braided string of considerable strength, are inclosed to- 
gether in a spiral wrapping of spring brass wire. Over this wrap- 
ping, which extends throughout the entire length of the cord, are 
placed two layers of linen braiding extending over the whole length 
of the cord; a third, or reinforcing braiding, also extending for 
about a foot from the plug end of the cord. One of the conductors 
is provided at the plug end with a metallic clip, which, by means of 
a small machine screw and washer, is fastened to the tip conductor 
of the plug, which extends back into the opening within the shell. 
The sleeve conductor of the plug extends in the form of a hollow 
plug to the extreme rear portion of the plug, where it is provided 





FIG. 166.— CORD AND PLUG. 



with a very coarse internal screw-thread, into which the braiding of 
the cord, after being wrapped with linen thread, is tightly screwed 
for the purpose of fastening the plug to the cord. The sleeve con- 
ductor of the cord is merely bent back over the wrapped end of the 
cord, and this makes contact with the sleeve conductor of the plug, 
being held in place by the pressure of the braiding against the in- 
ternal screw thread of the sleeve. 

In Fig. 167 is shown in diagrammatic form the circuits of a 
switch-board of this class. Here the line wires, I 1 and I 2 , forming 
the two sides of a metallic circuit, enter the spring-jacks, c, c 1 , and c 2 , 
in the manner described in connection with Fig. 162. It will be 
noticed that while the tip-spring, d, is in its normal position, circuit 
is traced from the line, I 1 , through the coil of the drop. /„ and back to 
line / 2 , so that current sent from the subscriber's station will actuate 
the drop, thus indicating a call. When one of the plugs, P or P\ is 
inserted into the jack spring, d, is raised from its normal resting- 



186 



AMERICAN TELEPHONE PRACTICE, 



place and breaks contact with the terminal leading to the drop- 
coil, thus cutting the drop out of the circuit. At the same time, 
the connection is continued from the two line wires, I 1 and I 2 , to the 
two strands of the cord circuit. When an operator notices that a 
drop has fallen she inserts the answering plug, P, into the jack cor- 
responding to that drop, and by pressing the button, K, belonging 
to that cord circuit, bridges her telephone set, T, across the two 
strands, i and 2, of the cord circuit. This enables her to communi- 
cate with the subscriber calling, to ascertain his wants. She then 
inserts the calling plug, P', into the jack of the called subscriber and 
presses the button, K', thus connecting the terminal of the generator, 
G, with the two sides of the line of the subscriber called. 

It will be noticed that when the key, K', is in its normal posi- 
tion the conductors from the tip and sleeve of the answering plug 
to the tip and sleeve of the calling plug are made continuous by 
the springs of the calling key resting against their inside anvils. 




FIG. 167.— SWITCH-BOARD CIRCUIT FOR METALLIC LINES. 



When the key is depressed the springs break contact with the inside 
anvils, thus severing the connection between plugs, P and P' t and 
immediately afterward connect with the outside anvils forming the 
terminals of the generators, G, thus sending current over the called 
subscriber's line. 

The clearing-out drop, C 0, is permanently bridged across the 
cord circuit as shown, in order to indicate to the operator when 
either subscriber rings off. In order that the efficiency in talking 
may not be impaired, this drop is made of high resistance and high 
impedance. 

The line-drops may be of the ordinary type described in connec- 
tion with the grounded-circuit switch-board. The clearing-drops, 
however, must be made to meet more difficult requirements than 
the line-drops. As they are always bridged across the circuit of 
two connected subscribers, it is found that unless special precau- 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 187 

tions are taken much trouble will be experienced from cross-talk 
due to induction between two adjacent drops. This difficulty can- 
not be overcome as in the line-drops, by cutting them out of the 
circuit whenever two subscribers are connected, inasmuch as the 
very purpose for which they exist requires them to be always in 
such circuits. Neither can it be overcome by placing the drops at 
such a distance from one another that this induction will not be 
felt, for the limited space on switch-boards requires that they be 
put as close together as mechanical conditions will allow. 

It has thus been found necessary to design a drop which would 
neither affect nor be affected by any similar drop in its immediate 




FIG. 168.— THE WARNER TUBULAR DROP. 



vicinity. This has been accomplished in several ways, but the best 
example is that shown in Fig. 168, which illustrates what is known 
as the "Warner Drop." In this the coil is wound in the ordinary man- 
ner on a soft-iron core and is then encased in a tubular shield, c, 
also of soft iron. The armature, d, is pivoted at points, c, in a 
bracket, f, mounted directly on the rear portion of the tubular 
magnet. From this armature, a rod, a, extends forward through 
a notch in the front plate, b, in such manner as to engage the upper 
portion of the shutter and thus hold it in its raised position. A 
screw, /, passing through the front plate, /\ serves not only to hold 



188 



AMERICAN TELEPHONE PRACTICE. 



the magnet in place, but to hold the core in its place within the shell. 
The terminals of the coil are led out through two small holes in the 
armature, and are connected with the terminals, h i, mounted on an 
insulating strip, carried on the bracket, f. 

These drops should be so nicely made that the armature, d, will 
fit closely against the end of the tube, c, in such manner as to almost 
completely close the magnetic circuit in which the coil is placed. The 
lines of force generated by the passage of a current through the 
coil follow almost entirely the path provided for them by the shell 
and the core of the magnet, thus not only producing a very efficient 
electromagnet, but also preventing any of the lines of force from 
extending beyond the limits of the shell. These drops are usually 
wound to a resistance of 500 ohms, and may be mounted as closely 




FIG. 169.-STRIP OF TEN DROPS. 



together as desired without producing perceptible cross-talk. The 
impedance due to the great number of turns in the coil, and to the 
perfect magnetic circuit surrounding the same, is so great that prac- 
tically no diminution in the strength of speech transmission is felt 
due to its being bridged across the line. 

In the modern construction of the Warner Drop, instead of the 
rearwardly extending bracket being provided for carrying the ter- 
minals of the coil, these terminals are made in the shape of heavy 
pins of brass, firmly screwed into the rear head of the coil, and to 
these pins the terminals of the coil winding are soldered. These 
pins project through the armature of the coil in substantially the 
same manner as in the original Warner Drop, and form terminals of 
sufficient strength to need no outer support. 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 189 

The Warner type of drop has proved so satisfactory and is of such 
compact construction that it has been adopted for use as line, as 
well as clearing out purposes. 

For use as both line and clearing out drops it has become com- 
mon practice to mount a number, usually ten, of these drops on a 
single strip; this strip being provided with means for securing 
it into the framework of the switch-board, and forming a common 
support for all of the drops on it. Such a strip of 10 drops is shown 
in Fig. 169. 

Instead of mounting the jacks individually, as in the types so far 
considered, it is common practice to build them in strips, usually of 
10 or 20, such a strip, comprising a number of jacks, being an in- 
tegral piece of apparatus, no means being provided for separating 
the various jacks from the strip without entirely dismembering their 
parts. Such a strip comprising 10 jacks is shown in Fig. 170. In 



43te', 



■a M, 



FIG. 170.-STRIP OF TEN JACKS. 

this the front strip is of hard rubber, having ten holes drilled in its 
face, into which are inserted the sleeve contacts, these being of 
German silver or brass. Projecting from the rear of each sleeve, 
and, in fact, made of the same piece of metal, is a connection shank 
which extends back through the hard rubber strip forming the rear 
of the jack framework, where it ends in a suitable clip, to which the 
wire may be soldered. The other contacts of the jack are mounted 
in the rear strip only, each of them being provided with clips pro- 
jecting rearwardly from this strip for the purpose of soldering the 
connecting wires. Such a strip of jacks is provided with means for 
fastening it rigidly in the iron framework of the switch-board. This 
construction has many advantages over the individually mounted 
jack, the principal ones being that of greater rigidity and economy 
of space. The spacing of the jacks in this strip is the same as that 
on the drops shown in Fig. 169, and therefore these jacks may be 



.90 



AMERICAN TELEPHONE PRACTICE. 



used in connection with the drops, being mounted directly above or 
below them in the same switch-board cabinet. 

The ringing and listening keys, by which the operators generator 
or telephone may be connected with the circuit of any line through a 
cord circuit, have assumed a great variety of forms, only a few types 




FIG. 171.— RINGING AND LISTENING KEY. 

of which have survived. One of the most common types is shown 
in Fig. 171, in which is represented the under side of the key shelf. 
The handle by which the key is operated projects through the 
shelf, and is shown at the top of the cut. This handle is pivoted 
within the shelf, and operates a ball cam in a manner to cause it to 
slide between one or the other pairs of springs, between which it nor- 
mally rests. This is called a combined ringing and listening key, 
the act of ringing being accomplished by pressing the handle to the 
left, causing the ball to move to the right, while the action of listen- 
ing is caused by a reverse movement of the handle, causing the cam 
to move between the left-hand set of springs. The circuit connec- 
tions of such a key are shown in Fig. 172, where P and P' are re- 
spectively the answering and calling plugs. The tip and sleeve 
strands of the answering cord are respectively connected to the 




FIG. 172.— CIRCUIT OF COMBINED RINGING AND LISTENING KEY. 



springs 1 and 2, and also to the springs 3 and 4. Normally resting 
against the springs 3 and 4 are the springs 5 and 6, which are per- 
manently connected with the tip and sleeve strands, respectively, of 
the calling cord. The springs 7 and 8 form the terminals of the 
operator's telephone set, while the springs 9 and 10 form the termi- 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 191 

nals of the calling generator. It will be seen that the tip strand is 
normally continuous from the tip of the plug, P, to the tip of the 
plug, P', through the springs 3 and 5 of the ringing key; likewise 
the sleeve strand is normally continuous from the springs 4 and 6 
of this key. If the ball cam, A, is pressed to the left by means of 
the cam handle the operator's telephone is bridged across the cir- 
cuit by causing the springs I and 2 to make contact with the springs 
7 and 8, respectively. If the ball cam is pressed to the right the 




FIG. 173.-DIAGRAM OF LISTENING KEY. 

connection between the plug P and P' is entirely severed by the 
breaking of the contact between the springs 3 and 5 and 4 and 6 
respectively. A little later in the movement of the cam the springs 
5 and 6 make contact with the springs 9 and 10, thus connecting the 
generator terminal with the tip and sleeve of the calling plug. The 
reason for breaking the connection between the two plugs in the 
ringing operation is to prevent the current from the calling gener- 
ator also being sent out on the line of the calling subscriber, with 
which the plug, P, is connected. If this happened the current would 
be likely to traverse the receiver coil held to the ear of the waiting 
subscriber, giving him what is commonly known as a "ring in the 
ear," a decidedly unpleasant and sometimes dangerous experience. 
In diagrammatic illustrations of cord circuits it is usually more con- 




FIG. 174.— DIAGRAM OF RINGING KEY. 

venient to show the ringing and listening keys separately, although 
they may, in fact, form virtually one piece of apparatus. Thus Fig. 
173 would represent the listening key, and Fig. 174 the ringing key. 
It is also very common in circuit diagrams to omit entirely the ball 
cam, and also to omit the supporting blocks upon which the springs 
are mounted. 

It may be said in general that in diagrammatic illustration of tele- 
phone circuits the details of mechanical construction must often be 



192 



AMERICAN TELEPHONE PRACTICE. 



sacrificed entirely to clearness, in order to represent the circuit in an 
intelligible manner. The telephone circuit is often such a compli- 
cated thing that it should not be required to carry with it any 
degree of accuracy as to mechanical construction or arrangement. 

It is frequently desirable to provide in connection with the cord 
circuit what is commonly called a ring-back key, by means of which 
the operator may send a calling current out on the line of the calling 
subscriber, i. e., over the answering plug. This feature is not, how- 
ever, thought to be desirable except in special cases, because ring- 
ing of the calling subscriber is not, as a rule, necessary, and if 
it is necessary the operator may always accomplish it by changing 
plugs, that is, by placing the calling plug into the jack of the calling 
subscriber. This disadvantage, on account of the comparatively 
rare necessity of ringing the calling subscriber, is not sufficient to 
warrant the disadvantage in the point of complexity and expense 
of having an extra set of ringing key springs. Such an arrange- 



Gen. 

1 



1™* I 



1 




FIG. 175.— CORD CIRCUIT WITH RING-BACK KEY. 



merit, however, is shown in Fig. 175, where an extra ringing key is 
provided in connection with the answering plug. 

In diagrams of cord circuits, in order to avoid the complexity 
caused by showing the generator and telephone set of the operator, 
it is often sufficient to indicate these in some such manner as is 
shown in Fig. 175. A still better way of indicating the connection 
of the generator is to label the leads from the ringing key to the 
generator with the algebraic "plus or minus" sign (-) ), this in- 
dicating that the leads run to a source of current which alternately 
changes its sign or direction. This symbolic representation will be 
largely used in this work hereafter. 

It frequently becomes necessary to connect grounded to metallic 
circuit lines in switch-boards, and in order to do this without un- 
balancing the metallic circuit line the best way is to have a certain 
number of cord circuits equipped with repeating coils in such man- 
ner that the voice currents coming over one of the line circuits will 



MAGNETO SWITCH-BOARD TOR SMALL LXCHANGLS, 



193 



pass through one winding of the repeating coil, thereby inducing 
similar currents in the other line circuit through the medium of the 
other winding of the coil. Such an arrangement is shown in Fig. 
176, where a metallic circuit line is shown connected with a 
grounded line. It is evident that the voice currents in the metallic 




FIG. 176. -CORD CIRCUIT FOR GROUNDED AND METALLIC LINES. 

circuit line will pass through winding, 1, of the repeating coil in the 
cord circuit, thereby inducing similar currents in winding, 2, which 
will flow through the circuit of the grounded line. This circuit may 
be traced from ground at the central office through the sleeve con- 
tact w T ith the jack and plug through the winding, 2, of the induction 
coil, thence through the tip of the plug and jack to line and to 
ground at the subscriber's station. 

The only objection to the plan shown is that it is sometimes neces- 
sary for the operator to change cords ; for instance, if she had 
answered with the plug belonging to the pair of metallic circuit 
cords and found that the call was for a grounded line, she would 
have to change the answering plug, using one belonging to the com- 
bination pair of cords, as shown in Fig. 176. In order to obviate 
this, what is termed a repeating coil key is sometimes used in con- 
nection with all the cord circuits, this key being similar in con- 



j^r^m 




Drop. 

**4 



Call. 




FIG. 11 



REPEATING COIL REV IX CORP CIRCUIT 



struction to the ordinary ringing key, except that it is adapted to 
lock in either direction. This arrangement is shown in Fig. 177. 
in which, besides the regular cord circuit having a ring-back key, 
an additional key, k, and a repeating coil are provided. With the 
key in the position shown, it will be seen that the repeating coil is 



194 AMERICAN TELEPHONE PRACTICE. 

cut out, the tip and sleeve strands of the cord circuit being con- 
tinuous from the answering to the calling plug. With the key 
thrown in its opposite position, however, that is, with the springs I 
and 2 in contact with the springs 3 and 4, instead of with the springs 
5 and 6, it will be seen that the answering plug is completely sepa- 
rated from the calling plug, the circuit of the answering plug being 
traced from the tip of this plug through the winding, A, of the re- 
peating coil to contact, 3, of the repeating coil key, thence to contact, 
1, and back to the sleeve of the plug. The circuit of the calling 
plug may be traced from the tip of this plug to the spring, 2, thence 
to the spring, 4, and through the winding, B, to the repeating coil, 
back to the sleeve of the calling plug. It will be seen that with this 
arrangement the circuit assumes the same aspect as that shown in 
Fig. 176. 

Very few switch-boards are now made having single contact 
plugs and jacks, as described in connection with Figs. 158 to 162, 
inclusive. By this is meant that few boards are now constructed 
that are adapted to grounded or common return lines only. A 
board for metallic circuit lines is equally well adapted to serve 
common return or grounded lines, in which case that side of the 
jack which is not connected to line is grounded. Thus, if all the 
sleeve contacts of the jack are connected with earth, as shown at the 
right-hand side of Fig. 176, it is evident that the talking circuit will 
be established through the tip side of the cord circuit, the sleeve 
side of the cord circuit being practically dead, where repeating coils 
are not used. 

It is generally considered of great advantage to have switch- 
boards so arranged that it will be unnecessary for the operator to 
manually restore the drops. The reason for this is that every move- 
ment on the part of the operator, in establishing a connection be- 
tween two subscribers, requires a certain amount of time, and that 
in the busier portions of the day an operator is worked almost to 
the extremity of her endurance, and therefore that the saving of 
any movements in handling these connections will be a gain in the 
rapidity with which the board can be operated. Such saving of the 
work of the operator not only insures a quicker and therefore a 
better service, but also may reduce the cost of the operation of the 
exchange by enabling fewer operators to handle -the system. There 
are, however, some who contend that the greater part of an oper- 
ator's time is necessarily taken up in talking or listening to the sub- 
scriber in order to ascertain his wishes, and that while she is doing- 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 



195 



this she may restore the drops by hand without loss of time. Not- 
withstanding this, however, the number of exchanges using self- 
restoring drops is rapidly increasing, and many inventions have 
recently been made and put into practice to bring about this result. 

Nearly all of the so-called independent manufacturing com- 
panies are supplying ''combined drops and jacks," the drop and 
jack being associated so as to form a single piece of apparatus. In 
these the drop is so arranged as to be restored by some mechanical 
movement brought about by the insertion of a plug into the cor- 
responding jack. The Bell companies, where they use "self-re- 
storing" drops (which they seldom do in small boards), accom- 
plish the result electrically, a restoring coil being placed on the drop, 
to retract the shutter when energized by the insertion of a plug into 
a jack. 




FIG. ITS.— BELL SELF-RESTORING DROP. 

In Fig. 178 a side elevation and partial section is given of the elec- 
trically restoring drop of the Bell companies, this being the product 
of the Western Electric Company. In this, a, is a tubular electro- 
magnet, carrying on its rear end an armature, C, pivoted at c, which 
armature carries an arm, r\ which projects forward and is pro- 
vided with a catch, c 4 , on its extremity. So far the arrangement is 
almost identical with that of the Warner tubular drop already de- 
scribed. A second tubular electromagnet, d, is secured to the front 
of the mounting strip, b, which also supports the magnet, a. This 
second magnet has its poles facing the front of the board. An arm- 
ature, e, is pivoted at its lower side. The catch, c 4 , on the rod, r\ 
is adapted to engage a lug, e 3 , on the armature and retain it in its 
vertical position. Pivoted on the bracket, /, which is insulated from 
the magnet, is a light shutter, g, of aluminum. The tendencv of the 
armature, e, when released is to fall outward, and in so doing it 
presses against the light shutter, g, just below its pivotal point, and 
forces it into a horizontal position. 



196 



AMERICAN TELEPHONE PRACTICE. 



The coil of the electromagnet, a, is usually termed the line 
coil, and is included in the circuit of the line wire. The coil of 
the electromagnet, d, termed the restoring coil, is in a local circuit 
containing a battery which is closed by the insertion of a plug into 
the spring-jack of the line belonging to that drop. 

Various arrangements associating drops of this type with the 
line circuits and with the local circuits at the switch-board have been 
devised and put into practical operation, the arrangement in Fig. 
179 being typical. The actuating coil, a, is permanently bridged 
across the two sides of the line wire, it being wound to a resistance 
of about 500 ohms to prevent short-circuiting the voice current. 




FIG. 179.-BELL SELF-RESTORIXG DROP. 



Two sleeves or thimbles, k k 1 , are shown on the jack, the inner one, 
k, being connected permanently to ground through a battery, k 3 . 
The outer thimble, k 1 , is connected to the ground directly through the 
restoring coil, d. When with this arrangement a plug is inserted, 
the two thimbles of the jack are short-circuited by the sleeve on the 
plug, and the circuit through the restoring coil is thus closed through 
the battery, k 3 . This pulls the armature, e, back until it engages the 
catch, c\ and thus allows the shutter to swing into its normal posi- 
tion. 

The combined drop and jack, when properly made, possesses some 
advantages not to be found in the electrically restoring type of 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 



197 



drop. In the first place, all the additional coils on the drops, and 
the additional contacts on the spring-jacks and the additional wiring 
between the two, are entirely done away with. Another advantage, 
and one that is usually overlooked, is that when the drop falls the 
eye of the operator is attracted directly to the point into which she 




FIG. ISO.— WESTERN COMBINED DROP AND JACK. 

must insert her plug; while in the forms where the jacks and the 
drops are entirely removed from each other the operator must first 
look at the drop, ascertain its number, and then look for the corre- 
sponding number of jack on the board below. This very materially 
increases the ease of operation and consequently tends in itself to 
give more rapid service. 

One of the first mechanically, "self-restoring" drops to be put inlo 




FIG. 1S1.-WESTERN COMBINED DROP AND TACK. 



successful commercial operation was that of the Western Telephone 
Construction Company. It is shown in Figs. 180 and 1S1. the former 
figure showing the shutter in its normal position, ami the latter, after 
it has been thrown down by an incoming call current. In these 
figures the arrangements are such that the spring-jack lies directly 



19S AMERICAN TELEPHONE PRACTICE. 

in front of the actuating coil, E, and the shutter, C, is so arranged 
as to fall directly in front of the jack when released by the arma- 
ture, F. The combined jack and drop are mounted on a base of hard 
rubber, A. The armature, F, is pivoted in the front head, H, of 
the electromagnet by the pivot screw, P, and has a forwardly ex- 
tending arm adapted to support the shutter, C, in a horizontal posi- 
tion. A small leaf spring, S, normally holds the rear end of the 
armature away from the rear head, H, of the coil and in a position 
to be attracted by that head when a calling current is sent through 
the coil. The attraction of the rear end of the armature causes its 
front end to move sidewise and release the shutter, thus allowing it 
to fall into a vertical position and display itself to the view of the 
operator, as shown in Fig. 181. 

In order to make conection with the line the operator inserts her 
plug directly against the shutter, which is down, and in so doing re- 
stores the shutter to its normal horizontal position by the direct 
thrust of the plug. The plug is guided into its jack by the shield or 
guide-plate, K. In entering the jack the spring, /, is lifted off the 
anvil, /, by the sleeve of the plug, thus breaking the connection 
through the coil of the drop. The spring, /, makes contact with the 
sleeve of the plug, while the spring shown on the under sides of the 
jack makes contact with the tip, thus continuing the two sides of the 
line to the two strands of the cord. 

These drops and jacks are mounted into a sort of an tl egg case" 
composed of the bases, A, and the hard-rubber side-pieces, B B. 
These egg cases usually contain one hundred compartments, ten 
wide and ten high; ij inches is allowed in each direction between 
the centers of the jacks. The wires on which the shutters are hung 
are common to each horizontal row of ten, and the other wire shown 
is also common to each row of ten. These two wires form the ter- 
minals of the night-alarm circuit, and when a shutter is down the 
lug, D, on the shutter strikes against the rear wire, thus making con- 
nection between the two and causing the night bell to ring. 

This combined jack and drop has given good service, but has sev- 
eral rather serious faults, chief among which is the fact that the tip 
and sleeve-springs are too short and therefore liable to lose their 
tension. Also with the "egg case" construction repairs are almost 
impossible without taking down the entire structure. These defects 
have been to a large extent removed in a more recent form of ap- 
paratus put on the market by this company, and designed by Mr. A. 
M. Knudsen. In this, shown in Fig. 182, the general arrangement 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 



199 



of the various parts is the same as in the type just described, but the 
springs are made longer by mounting them upon the sides of the 
jack base, and in fact making them continuations of the frame itself. 
The rear portions of these springs are provided with thumb-screws 
carrying thumb-nuts, I and 2, which pass through a back panel in the 
board and secure the entire drop and jack in position, and at the 
same time afford means for connecting with the tip and sleeve sides 




FIG. 182.— WESTERN IMPROVED DROP AND JACK. 

of the line. The jack-tube, A, and the shield for guiding the plug 
into the socket are formed from a single casting of brass firmly 
secured to the jack base, M, thus providing a much more rigid con- 
struction than that shown in Figs. 180 and 181. The shutter, E. 
operates in the same manner, it being shown in its exposed posi- 




FIG. 1S3.-SIDE ELEVATION AMERICAN DROP AND TACK. 



tion, the path through which it swings being indicated by the 
curved dotted line. Repairs on these drops may be readily made, 
as the entire structure of any drop and jack may be withdrawn from 
the front face of the board by removing the thumb-nuts. 1 and 2, 

Great sensitiveness can never be attained with this drop, because 
the shutter rests upon the armature rod in such manner as to bear 



200 



AM ERIC AX TELEPHOXE PRACTICE. 



upon it with its entire weight. It can therefore only be released 
by a considerable effort on the part of the armature, this effort being 
due to the friction between the shutter and the armature rod. 

Another form of mechanically self-restoring drop is that now 



\Mc 



inmmmwmmmmmmn 



I 



H«iu.tt»mimrtmi»ttmiw« 




FIG. 1S4.— TOP VIEW, WTTH COIL REMOVED. 

manufactured by the American Electric Telephone Company. In 
this drop, which is shown in Figs. 183, 184 and 185, the actuating 
coil is mounted directly above the spring-jack. The coil is inclosed 
on the sides in a sheet-iron frame or box, C, for lessening the amount 
of induction between adjacent drops. The armature of the magnet 
is pivoted at the rear of this shield, and carries a forwardly project- 




FIG. 1S5.— TOP VIEW, WITH COIL IN PLACE. 



ing lever, /, which in turn carries on its forward end a catch for hold- 
ing the shutter, d, in its vertical position. On the shutter is placed a 
cam, c. which, when the shutter is down, lies in front of the opening 
of the jack. The plug shown in Fig. 186 carries an enlargement or 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 201 

collar, k, which collar engages the cam, c, on the shutter when the 
plug is inserted into the jack, and forces the shutter into its normal 
position. 

No cut-out is provided for the coil, which is therefore left in 
series in the line during a conversation. The coil therefore serves 
as a clearing-out drop, and when so actuated the cam on the shutter 
falls in front of the collar on the plug. When, therefore, the plug 
is withdrawn from the jack, after the clearing-out signal has been 
sent, the cam again engages the collar on the plug and the shutter 
is restored again to its vertical position. 

The entire structure of the combined drop and jack is removable 
from the board by taking the thumb-nuts off the screws shown in 
the rear. These screws pass through the board forming the frame 
of the switch-board, and serve not only to hold the jack and drop 
in place, but to establish a connection between the line wires and 
the line springs of the jack. Small springs, g g, on the back of the 
jack register with corresponding contacts on the front side of the 
backboard, thus serving to extend the night alarm and generator cir- 




FIG. 186.— AMERICAN PLUG. 

cuits from the jacks to the other parts of the switch-board. By this 
means the proper connections are automatically made when the jack 
is slipped in place. 

The drop illustrated in side elevation in Fig. 183 is of the com- 
mon-return type, and is therefore provided with but one line 
terminal. Figs. 184 and 185 show a later pattern adapted to 
metallic circuits and operating in the same general manner so far 
as the restoring of the shutter is concerned. Fig. 184 is a horizontal 
view of the annunciator removed, showing the arrangement of the 
various parts of the jack. In this figure the coil is indicated at C 
in order to better illustrate its circuit connections. Fig. 185 is a 
similar view of the complete apparatus, with the annunciator in 
place. The various circuits of the apparatus will be understood 
most readily by considering Fig. 184. In this m is the jack-tube 
which is directly connected with the line terminal screw. /.. This 
tube is provided with a spring, ;/, which serves to establish a firmer 
contact with the sleeve of the plug when inserted into the jack. 
The coil, C, of the annunciator is connected directly between the line 



202 AMERICAN TELEPHOXE PRACTICE. 

terminal screw, L', and the tip-spring, p, the sharply bent portion of 
which spring is adapted to make contact with the tip of the plug 
when inserted into the jack. A spring, q, is connected by means 
of one of the small springs, g, in Fig. 183 to one terminal of the 
generator. This spring, q, is provided with a metallic pin which 
projects through a hole in the jack-tube, m, to a sufficient distance 
to make contact with the enlarged sleeve of the plug when the latter 
is inserted into the jack to its fullest extent, but not far enough to 
engage the tip contact when the plug is in its normal position in the 
jack. By this means, when the plug is inserted as far as it will go 
into the jack, one terminal of the generator is connected with the 
line terminal screw, L, by means of the sleeve spring, n, and the 
generator spring, q, both coming in contact with the sleeve of the 
plug. The other terminal of the generator is connected with the 
spring, 7, through the medium of one of the small contact springs, 
g, on the back of the jack. Upon pushing the plug as far as it will 
go into the jack, the tip-spring, p, rides upon the insulated portion 
of the plug, thus pressing the thin spring, s, which lies parallel with, 
but is insulated from, the tip-spring, into engagement with the 
generator spring, j. This connects the line terminal screw. L', with 
the generator spring, /. through' the medium of the strap conductor, 
k, and the calling current is therefore sent to line. It will be no- 
ticed that the path by which the generator current passes to line is 
not through the coil of the annunciator, but through the strap, k, 
instead ; and it will also be noticed that the tip conductor of the plug 
is disconnected from the tip-spring, p, before the contact is made 
with the generator-spring. /, and therefore no calling current will 
pass back over the cord circuit through the operator's telephone. 
Upon removing the pressure from the plug, a coiled spring in its 
handle forces it out of the jack for a short distance until it assumes 
the normal or talking position. 

In later practice the two plugs of a pair are provided with a long 
and a short tip respectively, so that one of the connected lines will 
have its coil shunted out of circuit, the other being left in as a clear- 
ing out drop. 

In Fig. 187 is shown several views of a combined drop and jack 
designed by the writer, this being the device used in the magneto 
switch-board of the Kellogg Switchboard & Supply Company. The 
annunciator consists of an ordinary tubular drop mounted upon but 
insulated from the upper portions of the brass strip, C. The jack is 
mounted immediately below the drop and consists of a sleeve, A, 



MAGNETO SWITCH-BOARD TOR SMALL EXCHANGES. 



203 



and a frame, B, both of brass, the sleeve, A, being in the form of 
a hollow hexagonal-headed screw. The sleeve passes through the 
mounting strip and engages a thread in the frame, B, clamping the 
jack firmly on the mounting strip, C, so that there is no possibility 
of short circuits between the jack and the mounting strip. The tip- 
spring, B, of the jack has a forwardly projecting lug which passes 
through the hole in the mounting strip and is adapted to engage 
the shutter when down. The insertion of the plug into the jack 





FIG. 187.— KELLOGG COMBINED DROP AND JACK 



lifts this spring, B, out of contact with the spring, E, with which it 
normally engages, and as this spring rides upon the tip of the plug 
its forwardly projecting portion engages the drop, thus restoring 
it to its normal position. 

The entire electromagnet of the drop may be removed from the 
board indepndently of the jack and the jack may also be removed 
independently of the drop. 

One of the line terminals is formed by a rearwardly projecting 



204 AMERICAN TELEPHONE PRACTICE. 

lug which is a part of the casting of the frame, B, while the other 
terminal is formed by a similar lug, /, fastened directly on the line 
spring, D. The drop coil is connected between one of the line ter- 
minals and the spring, E, so as to be cut out of circuit by the inser 
tion of the plug. 

Five of these combined jacks and drops are usually mounted on 
the common plate, C, as shown in Fig. 188. The front of this plate 
is covered by a heavy hard rubber strip serving as part of the in- 
sulation used in separating the drop and jack from the metallic 
mounting plate and also giving a finish to the front of the strip. 
The night alarm wire is carried on the front of this strip and the 
shutter in falling serves to press a contact spring secured on the 







FIG. 188.— STRIP OF KELLOGG COMBINED DROPS AID JACKS. 

inner face of the mounting plate into engagement with this wire 
to close the night alarm circuit. A better idea of the construction 
of one of these drops may be had from the perspective view of the 
rear of one of them, shown in Fig. 189. 

Many other forms of mechanically self-restoring drops have been 
devised, but the types here described have come into by far the most 
general use. 

As an illustration of the saving which either the electrically or 
mechanically self-restoring drops bring about in the operation of 
switch-boards, certain boards may be cited that have been put into 
use where the drops are of the ordinary hand-restoring type, placed 
in series in the line and not cut out by the insertion of the plug. 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 205 

In the establishing and tearing clown of a connection between two 
subscribers, the operator was required to restore a switch-board drop 
four different times. First she restored the drop of the line of the 
calling subscriber, next when she sent a calling current to the line 
of the called subscriber this current passed through the drop of that 
line, causing it to fall. This she also restored by hand, and lastly 
when one or both of the subscribers rung off, the drops of each line 
fell and were restored by hand, thus making four in all. Such 
switch-boards are, of course, necssarily slow. Moreover, they are 
almost invariably much larger and more cumbersome than the more 
modern types, but even these drawbacks have not interfered with 
their giving satisfactory service in some cases in small exchanges. 
It is sometimes desirable to have the line signal at a switch-board 




FIG. 189.— REAR VIEW KELLOGG DROP AND JACK. 

give an audible signal which will cease sounding as soon as the 
line current ceases. Of course, the ordinary magneto drop, if pro- 
vided with a night alarm attachment, may be made to give an 
audible signal, but this signal persists as long as the drop is allowed 
to remain down, regardless of whether or not the actuating current 
has ceased to flow through the coil of the drop. In small switch- 
boards sometimes employed in village exchanges, the amount of 
attention required at the board does not warrant the constant at- 
tendance of an operator. The lines running into such an exchange 
are frequently party lines of the bridging type, and more often than 
not a call on such a line is for another party on the same line. Such 
a call would throw the switch-board drop and bring the operator 
to the board, unless some means were provided for the operator to 



206 AM ERIC AX TELEPHOXE PRACTICE. 

distinguish between calls that required her attention and those that 
did not. Obviously the night alarm method of attracting the 
operator does not answer in this case, for with it all calls are alike 
at the switchboard. 

Frequently the magneto bell is used as a line signal in place of 
a drop. This serves admirably unless there are several lines to be 
so equipped, in which case confusion is liable to exist as to which 
bell sounded. This may be overcome by using a magneto bell and 
a drop both bridged across the line, the drop serving to indicate 
which bell sounded. This, however, makes two bridges across each 
line where one should suffice, which is often a disadvantage in case 




FIG. 190.— COMBINED DROP AND RINGER. 

of heavily loaded lines. To obviate this a combined magneto bell 
and drop is sometimes used. 

Two of these mounted on a common strip are shown in Fig. 190, 
the particular device illustrated being that of the Kellogg Switch- 
board and Supply Company. In this, as in types of other manu- 
facturers, the bell tapper carries a small conical latch which projects 
forwardly to engage the shutter when in its raised position. The 
first movement of the tapper in either direction releases the shutter, 
after which the tapper is free to play between the gongs. This de- 
vice is usuallv arranged to mount directly on the front of the switch- 
board, and for that purpose the line jack is often associated directly 
with the bell as shown in this figure. 

The principal circuits used in magneto switch-boards, and also 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 207 

many of the various pieces of apparatus, such as drops, jacks, ring- 
ing and listening keys, etc., which go to make up such boards, have 
been discussed. The matter of properly organizing these various 
parts to form a complete switchboard is not of less importance than 
the proper design and construction of the parts themselves. The 




FIG. 191.— STANDARD TYPE OF SWITCH-BOARD— FRONT VIEW. 



main points to be sought in assembling the separate pieces of ap- 
paratus into a complete switchboard, are that the arrangement may 
be such as to facilitate the work of the operator; that all parts liable 
to get out of order shall be readily accessible for repairs : that all 
wiring shall be systematically arranged in a manner that shall pre- 
clude as far as possible the possibility of short circuits, crosses, and 



208 AMERICAN TELEPHOXE PRACTICE. 

open circuits ; that the various pieces of apparatus, as well as the 
circuits in which they are placed, shall be free from inductive in- 
fluences upon or from the other adjacent apparatuses or circuits; 
that the framework and enclosing cabinet upon which the various 
parts are mounted shall not, by virtue of shrinkage or warping, 
affect the proper working of the apparatus; that the design shall be 
such as to prevent, as far as possible, the injurious presence of dust ; 
and lastly, and of least importance, it is desirable to make the entire 
structure of pleasing appearance. 

In Fig. 191 is shown a typical form of magneto switch-board, this 
being one of the Kellogg Switchboard and Supply Company's make, 
employing the combined drop and jack shown in Figs. 187 and 188. 
In this particular board these are arranged in three panels, each 
panel having room enough for ten strips of five combined drops and 
jacks, and a single strip of five clearing-out drops placed at the 
bottom. When equipped with the less number of strips, the space 
is blanked with a wooden strip, as shown in the lower portion of the 
right-hand panel, where a single strip of drops and jacks is omitted. 
The pairs of plugs rest in sockets on a horizontal shelf or table below 
the drop and jack space and in front of these on a hinged shelf are 
mounted the ringing and listening keys. This hinged shelf is 
usually called the key shelf, and the stationary portion behind it 
which carries the plugs, the plug shelf. In front of the key shelf 
at the right-hand side of the cabinet may be seen the crank of the 
operator's generator, while in a corresponding position at the left 
of the cabinet may be seen the operator's head telephone with its 
band, temporarily hung on the operator's "cut-in plug," which is 
inserted in the operator's "cut-in jack." By means of this cut-in 
plug and jack the operator is enabled to connect her head receiver 
with the remaining portions of the talking circuit, or to disconnect 
it in case she desires to leave the board. The operator's transmitter 
is shown hanging from an arm in front of the drop and jack space. 

In Fig. 192 the rear view of this same board is shown, this figure 
serving to give an idea of the arrangement of the apparatus and 
wiring, the rear enclosing panel being removed for this purpose. 
About the middle of this cut will be seen the cord circuits upon which 
the fixed ends of the flexible cords terminate, and to which the cord 
circuit wiring extends. The cord weights which serve to keep the 
cords taut may be seen at the bottom of this view, and just above 
them the connecting rack to which all the circuits of the drops and 
jacks are led, the connections being made on the front side of this 



MAGNETO SWITCH-BOARD TOR SMALT EXCHANGES. 209 

board. The clips on this board extend entirely through it, and 
therefore form a convenient means for connecting the circuits of 




FTG. 192.-STANDARD TYPE OF SWITCH-BOARD-REAR VIEW. 
the outside lines. A more common practice now is to extend the 
pairs of wires leading from the drops and jacks in a cable perhaps 



210 



AMERICAN TELEPHONE PRACTICE. 



io feet long, in which case the connections with outside lines are 
made at the extremity of this cable. 

It is often desirable to make a switch-board cabinet wide enough 
to accommodate more than one operator, and such a cabinet is shown 
in Fig. 193, this particular one being arranged with the idea of 
allowing room for two operators if the business of the exchange 
grows sufficiently to require two. At present, however, this board 
has only one operator's position equipped. From this view a better 
idea may be obtained as to the arrangement of the plugs and keys, 




FIG. 193.— TWO-POSITION SWITCH-BOARD. 



and it is also instructive as showing the method commonly used of 
arranging for a greater ultimate equipment of cords, plugs and keys 
than that required in the first installation. As will be seen, 15 pairs 
of cords and plugs are equipped in the switch-board of Fig. 193, 
while room is provided, and the holes drilled, for five additional sets 
at the right-hand portion of the key and plug shelf. In order to 
prevent the unsightly appearance of these holes they are covered by 
suitable blanks which may be easily removed when the apparatus is 
eventually installed. 

There are many different ways of mounting the operator's trans- 



MAGNETO S1V ITCH-BOARD FOR SMALL EXCHANGES. 211 

mitter upon the switch-board, one of which is shown in Figs. 191 
and 193. A somewhat different and better arrangement is shown 
in Fig. 194, where the bracket or arm is supported on an upright 
secured to the roof board of the switch-board cabinet. The flexible 
cords forming a portion of the circuit of the transmitter pass over 
pulleys at each end of the arm and through holes in the roof board 
where counter-weights serve to balance the weight of the trans- 
mitter. This arrangement allows for the vertical adjustment of the 
transmitter to suit the convenience of the operator. The transmitter 
may also be adjusted toward or from the operator by sliding the 




FIG. 194.— OPERATOR'S TRANSMITTER SUSPENSION. 



horizontal arm through its socket in the vertical parts which are 
clamped in position by the thumb-screw as shown. 

In Fig. 195 is shown an arrangement of the switch-board apparatus 
where the drops and jacks are separately mounted, this being one ol 
the recent boards of the Stromberg-Carlson Company. It will be 
seen that the jacks are mounted in strips of ten as are also the drops. 
the two being arranged in separate banks. 

Another board of the Stromberg-Carlson manufacture is shown 
in Fig. 196. This shows a two-position board of the type where the 
drops and jacks are separately mounted, both positions being fully 
equipped. In this the clearing-out drops are arranged just below 



212 



AMERICAN TELEPHONE PRACTICE. 



the line drops, there being one of these for each pair of cords and 
plugs. At the left-hand portion of this cut is shown a cabinet to 
which all of the wiring from the switch-board is led, and to which 
also the various line wires are adapted to be brought for the pur- 
pose of connecting them with the switchboard wires. In some cases 




FIG. 195.— STROMBERG-CARLSOX MAGNETO BOARD. 



the lightning arresters which form the connecting links between 
the outside and the inside wiring are often mounted in such a 
cabinet. 

The idea of constructing switch-board parts in units which may 
be removed or placed at will, and by means of which a switch-board 
may be added to without deranging the parts already installed has 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 213 

been gaining more and more in favor. This idea, however, is per- 
haps most fully exemplified in the switch-board of the Sterling Elec- 
tric Company, of Lafayette, Inch, which is shown in Fig. 197. In 
this switchboard the drops and jacks are mounted in strips of ten, 
the strips being vertical instead of horizontal. Each strip of ten 
drops and jacks also carries with it a portion of the plug shelf, to- 
gether with a pair of cords and plugs, ringing and listening key, 
clearing-out drops, and the necessary wiring. One of these units 
may be removed without disturbing any of the others, and con- 
versely it will be obvious that in order to increase the board by 




FIG. 196.— TWO-POSITTON STROMBERG-CARLSON BOARD WITH 
TERMINAL CABINET. 



ten lines at any time, it is only necessary to secure one of these units 
into the framework, this being accomplished by means of four 
machine screws. With this arrangement the proper proportion ( 10 
per cent. ) of the pairs of cord circuits to the line circuits is always 
maintained. 

In the Sterling board the drops arc not of the self-restoring type, 
but each strip of ten drops is provided with a rack adapted when 
raised to restore any of the drops that are down in that strip. This 
rack is provided with a convenient handle to enable the operator to 
raise it at the same time that she inserts the plug. 

In Fig. [98 is shown a type o\ switch-board cabinet sometimes 



214 AM ERIC AX TELEPHONE PRACTICE. 

used in small village exchanges, these cabinets being mounted 
directly on the wall of the room. In order to allow inspection of 
the wiring, such cabinets are usually hinged on their backboards so 
as to allow them to swing out from the wall, the cable leading to 



FIG. 197.— STERLING SECTIONAL BOARD. 

the apparatus within being provided with a knee or hinge to allow 
a sufficient movement of the cabinet. 

The particular switch-board shown in this cut is one manufactured 
by the Stromberg-Carlson Company, and, as will be seen, the lines 
are in each case equipped with a combined drop and ringer, these 
being equipment for 14 lines in all. 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 215 



In very small exchanges an ordinary wall telephone is frequently 
used as an operators set, the binding posts of this telephone being 
connected to a two-conductor flexible cord terminating in a two- 
conductor plug. When a call is received over any line, the attendant 
may answer by taking the telephone receiver off its hook, and in- 
serting the plug into the jack of the line on which the call was 
received. If the attendant desires to ring out on the line, she has 
only to insert the telephone plug into the jack of that line, and turn 
the generator crank of the wall set after having hung up the receiver. 




FIG. 198.— WALL TYPE SWITCH-BOARD WITH RINGERS. 

This arrangement has the advantage of simplicity and cheapness, 
and the fact that it is not capable of quite such rapid manipulation 
is not a disadvantage where only a few lines are to be handled. 

The idea in the previous paragraph of using an ordinary wall 
telephone for an operator's set is embodied in the device shown in 
Fig. 199, in which the telephone really serves for an operating set 
of two lines only, or more properly for the two ends of one line. 
In addition to this, the ringer of the telephone set may be made to 
serve as a signal on one of the lines. As will be seen the switch- 
board consists of an ordinary extension bell box containing a ringer 
and three jacks. The circuit for this outfit, which is called a "toll 
cut-in box," is shown in Fig. 200. In this line Mo. 1 terminates in 



216 



AMERICAN TELEPHONE PRACTICE. 



the tip and sleeve contacts of jack No. i. Line No. 2 similarly ter- 
minates in the tip and sleeve contacts of jack No. 2. With the two 
jacks in their normal conditions, the two lines are connected together 
with the ringer bridged across their circuit at jack No. 3. When 
a call is received over the toll line for such a station, the operator 
in response to the ring plugs into jack No. 2, thus cutting out the 
normally bridged ringer and placing her telephone set across the 
line. If it is found that the party calling is on line No. I, the at- 





FIG. 399.— TOLL CUT-IN STATION. 



tendant may, for the purpose of obtaining better conversation, plug 
into jack No. 1, thus connecting the telephone with line No. 1 and 
cutting off line No. 2. When this is done, line No. 2 is still able to 
signal this station, because the ringer in the extension bell box is 
left bridged across its circuit. Similarly the party at this telephone 
may talk or ring on either line to the exclusion of the other by in- 
serting the plug in the corresponding jack. If it is desired for any 
reason to leave the two lines temporarily separated, the plug may be 
placed in jack No. 2, in which case the ringer of the telephone set 



MAGNETO SWITCH-BOARD FOR SMALL EXCHANGES. 217 

will serve as a signal-receiving device. for line Xo. i, the ringer in 
the extension bell box then serving for line No. 2. 

These devices are used to a very large extent as intermediate 
stations on toll lines where it is necessary to place apparatus of the 
simplest possible nature on account of cheapness and ease of main- 
tenance. 

Owing to the multitude of wires and connections that necessarily 
occur in wiring the various parts of switch-board apparatus, it is of 
the greatest importance that the wiring be done with a proper regard 



iine J. 



line & 




FIG. 200.— CIRCUITS OF TOLL CUT-IN STATION. 



for systematic arrangement. For this reason the use of cables has 
come into almost universal adoption for this purpose, the various 
wires leading to the different parts of the switch-board being bunched 
into a compact mass, forming a cable. The individual wires are led 
cut from the cable at points most convenient for attachment to their 
various terminals. The wires used in this work are usually of No. 
22 B. & S. gauge copper, tinned in order to facilitate their being 
soldered to their terminals. The insulation usually consists of one 
layer of Tussah silk and one or two layers of cotton ; or sometimes 
two lavcrs of silk and one of cotton. Tf the wires are to be used in 



218 AMERICAN TELEPHONE PRACTICE. 

forming cables by band they are usually impregnated with beeswax 
and afterwards bound into a cable by means of a strong linen twine. 
If, however, the wires are to be used in a machine-made cable which 
is to have its ends properly formed for attachment to the switch- 
board, they are usually not saturated with paraffin but bunched into 
the proper form of cable, then wrapped with heavy manilla paper, 
the whole being covered with a braiding of cotton. This outer cov- 
ering may be and usually is impregnated with beeswax or with some 
so-called fireproof paint, according to the requirements of the 
service. 

With few exceptions all connections between wires and the ter- 
minals of apparatus should be soldered. The only cases where it is 
impossible to follow this rule are where there is a likelihood of 
the necessity to break the connection, as, for instance, to take a piece 
of apparatus out of circuit. Where such a connection is not sol- 
dered the wire should always be clamped firmly in place by means 
of a machine screw, this screw passing into metal so as to allow no 
possibility of its loosening. The fewer such connections exist, how- 
ever, the better, and the absolute reliability of a soldered joint should 
not, as a rule, be sacrificed to the greater convenience of a screw- 
made joint. In soldering the connections of switch-board and other 
telephone apparatus the use of flux containing acid should be care- 
fully avoided. The best flux for this purpose is resin and the most 
convenient form of applying it is in the form known as resin core 
solder. In this the solder is rolled into a hollow wire which is filled 
with resin. 



CHAPTER XIII. 

THE THEORY OF THE MULTIPLE SWITCH-BOARD. 

When the number of subscribers in an exchange exceeds ap- 
proximately 500 the form of switch-boards so far considered becomes 
inadequate. To afford room for the number of operators needed 
to properly handle all of the calls, the board must be made of consid- 
erable width, and thus becomes too wide for the operators to reach 
over its entire face. 

When the number of lines becomes greater than can be handled 
by one operator more operators are employed, and to each is appor- 
tioned the signals of as many lines as she can properly attend. Each 
operator therefore answers the calls of her particular group of lines. 
So far as the answering of calls is concerned, therefore, the division 
of work between the operators may be made in a satisfactory man- 
ner. Not so, however, with the completing of the connection with 
the subscriber called for — for his line may terminate on some remote 
portion of the face of the board, to reach which the operator would 
be required to leave her position and carry the calling plug to its 
jack. This might be possible if the cords were long enough, but it 
is not difficult to see that the arrangement would have serious dis- 
advantages. 

If a sufficient number of operators could be placed within reach 
of a switch-board of such size as to enable any one of them to reach 
its entire face, there would be no need for the multiple board, or for 
any of the expedients which are now necessary in order to construct 
a switch-board for a large number of lines. 

It is therefore the size of the operators, or more specifically, their 
width, that causes the difficulty where a large number of lines must 
be accommodated. It is not due to the engineer's inability to place 
the required amount of apparatus in a given space, as is the popular 
impression, for the terminals of many thousand lines can be placed 
within an area easily reached by a single girl of ordinary size. To 
make this clear, it would be an easy matter to place the jacks and 
signals for ten thousand lines in a vertical space, say five and a half 
feet long, and not too high for an operator to reach. If now, a breed 
of operators be thought of, so thin as to enable one hundred of them 
to stand in a row before such a board, and yet tall enough to have 

219 



220 AMERICAN TELEPHONE PRACTICE. 

the reach of our present type of operators, each one of these slender 
young ladies might be given the care of a group of one hundred 
lines, the calls of which she would answer, and at the same time be 
able to reach the jack of any line that might be called for. 

It is fortunate for switch-board manufacturers, and perhaps for 
the human race in general, that girls are made as they are. 

The fundamental object of the multitple switch-board is to place 
within the reach of every operator a line terminal or spring-jack for 
every subscriber's line entering the central office. The switch-board 
is divided into sections, each section being of such size as to allow 
an operator, without undue exertion, to reach over this entire sur- 
face. Such a section usually affords room for comfortably seating 
three operators before it. On each section are placed a number 
of line signals, by means of which a subscriber is enabled to attract 
the attention of the operator, and a corresponding number of line 
jacks, by means of which the operator may make the initial con- 
nection with the line in response to a signal. These jacks, which 
are associated with the line signals, are termed answering jacks. 

In addition to the line signals and answering jacks are provided 
what are termed multiple jacks, there being one of these on each 
section for every line centering in the office. 

To express this in a little different way, the various lines entering 
the central office are divided into suitable groups, the lines in each 
terminating in a group of signals and answering jacks. Each of 
such groups is arranged at a certain position of the multiple switch- 
board, before which position an operator sits. As many answering 
jacks and signals are provided at any operator's position as can 
be properly attended to by the operator at that position. In ad- 
dition to this, all the lines entering the office are carried to every 
section of the switch-board, each line terminating in every section 
in a multiple jack. Each operator is provided with a certain num- 
ber of pairs of cords and plugs. 

The operator seated at any position may, therefore, in response 
to the call indicated on one of her line signals, insert one plug of 
the pair into the corresponding answering jack, and having ascer- 
tained the number of the subscriber desired, may insert the cor- 
responding plug of the pair into the multiple jack of the subscriber 
called for. As every subscriber's line has a multiple jack at every 
section of the switch-board, any operator is able to complete by her- 
self anv connection called for over one of the group of lines, to the 
calls of which she attends. 



THEORY OF THE MULTIPLE SWITCH-BOARD. 



221 



The underlying principle of the multiple switchboard, as just 
stated, is very simple. In practice, however, the greatest complex- 
ity is met, but this is due largely to the great number of repetitions 
of a single circuit which may, in itself, be comparatively simple. 

The general operation of the multiple switchboard and the gen- 
eral scheme of its principle may perhaps be, better understood by 
reference to Fig. 201, which has no regard whatever for the actual 
circuits, but merely for the arrangement of some of the most essen- 
tial parts. In this, three lines are shown entering a switch-board 
from the left, each line passing continuously through the three or 
more sections of the board, and being provided on each section with 
a spring jack or terminal, m', m % or m 3 . Connected also with each 
line, at one only of the sections, is a signal, s', s 2 or s 3 , and an 
answering jack, a', a 2 or a 3 . From this it follows that the call of 
the subscriber on anv one line may be answered at onlv one of the 



line*!. 



iine*Z. 



Zieeizcn *J. 



Ttth 



\0* 
0,1 



V 



Section *Sk 




3eciicn *3 



% 



AS 



V 



FIG. 201.— ARRANGEMENT OF PARTS IN MULTIPLE SWITCH-BOARD. 

sections, since his line signal appears at one section only. It is also 
evident that since a multiple jack is provided for each line on every 
section, that a subscriber may be called from any section. It thus 
follows that any operator, while she may answer the calls of but a 
few lines (those having signals at her position), may complete a 
connection between one of these lines and any other line in the 
entire office. Each operator, as in the case of small switch-boards, 
is provided with a certain number of pairs of plugs, P P\ connected 
by suitable flexible cords, c, and auxiliary apparatus. It will 
be seen that at section 2 connection has been made between lines 
Nos. 2 and 3 by means of such a pair of cords and plugs, the an- 
swering plug P having been inserted into the answering jack a 3 , in 
response to the signal s 3 , and the connection having been made with 
line No. 2 by inserting the corresponding plug of the pair into the 
multiple jack m 2 of that line at that section. 

It is evident that the appearance of a jack for each line on each 



222 AM ERIC AX TELEPHOXE PRACTICE. 

section completely solves the problem of enabling the operator 
to make any connection without moving from her section, but a 
little further thought will show that a difficulty is thus brought 
about. The presentation of a number of connection points belong- 
ing to a single line, to different operators located at the several sec- 
tions of the board, makes possible more than one connection with 
any line at the same time. For instance, while line No. 3 is con- 
nected with line Xo. 2 at section Xo. 2, line No. 3 might also 
be connected with line Xo. 1 at section No. 1, and it might also be 
connected with any other line at any other section. Such a condi- 
tion would, of course, bring about confusion. 

In order to prevent more than one connection being made with 
any line at the same time, what is termed the "busy" test has been 
provided, by means of which, as soon as any line is connected with 
another at any section, its electrical condition is so changed that 
an operator at any other section in attempting to make a connection 
with it will be given warning of the fact that a connection had al- 
ready been made with it. This warning is usually given by means 
of a click in the operator's head telephone, given when she attempts 
to make connection with a line already switched at another section. 

The size of the section is limited by the reach of the operator, and 
it is this fact that places a well-defined mechanical limit to the size 
or capacity of an ordinary multiple switch-board. Since each sec- 
tion- is to contain as many multiple jacks as there are lines center- 
ing at the office, it follows that the size of the jack determines the 
number of lines that can be placed on any section, and therefore 
the number of lines that can be served by the multiple board. Until 
quite recently it has been found impracticable to buitd jacks occupy- 
ing less space on the face of the board than one-half inch in both 
horizontal and vertical dimensions. This limited the number that 
could be placed within the reach of an operator to approximately 
6000 and at most to about 7200. Later improvements have, how- 
ever, made possible a material reduction in the size of the jack, so 
that it is now possible to place approximately 25,000 jacks within 
the reach of a single operator, thus raising the ultimate capacity 
of a multiple switch-board to that number of lines. 

While the size of the jack and the reach of the operator determine 
the number of multiple jacks that can be put on any section, and 
therefore determine the ultimate capacity of a switch-board, the 
number of line signals that can be handled by any one operator 
determines the number of sections that must be provided in the 



THEORY OF THE MULTIPLE SWITCH-BOARD. 223 

switch-board. Thus in a central office having 6000 lines, if three 
operators could handle all of the calls originating in the entire 
office, there would be no need of more than one section, for the 
three operators located in front of it would be all for which room 
need be provided. As a matter of fact, however, the number of 
line signals that can be handled by one operator is very limited, and 
varies from perhaps 10, in the very busiest lines of a large commer- 
cial center, to about 400 in communities where subscribers make but 
few calls a day. Probably 100 lines for each operator is a fair aver- 
age in modern work, and on this basis 300 answering jacks and 
signals will be placed on each section to be handled by the three 
operators located at that section. For a multiple switch-board of 
6000 lines, therefore, there would be need for 20 sections, and, of 
course, on each of these 20 sections 6000 multiple jacks would be 
placed. In such a switch-board, therefore, while there would be 
6000 line signals and 6000 answering jacks, there would be 120,000 
multiple jacks. 

As showing how the number of jacks increases as the number of 
lines increases, consider a 24,000-line multiple switch-board instead 
of one of 6000 lines. In a 24,000-line board there would be need 
for 240 operators' positions, or approximately 80 sections, still re- 
taining the assumption that each operator would be able to handle 
the calls of 100 lines. Since each section would contain 24,000 
multiple jacks, the entire switch-board would be provided with 
eighty times this many, or 1,920,000 multiple jacks. Thus, in 
comparing the 6000-line and the 24,000-line boards, it is found 
that while the number of lines has been increased fourfold, 
in the larger board, the number of jacks has been multiplied by 
16. A little consideration will show that with the same assumption 
as to the number of subscribers to be handled by any operator, the 
number of multiple jacks will vary directly as the square of the 
number of lines served by any multiple switch-board. This is not 
quite an accurate statement, because the conditions are slightly modi- 
fied by the fact that multiple jacks are usually extended somewhat 
beyond the last operator's position at each end of the board for the 
purpose which will be pointed out later. 

In general, however, it may be said that the number of line sig- 
nals and answering jacks in a multiple board varies directly as the 
number of lines served, as do also the number of sections, the num- 
ber of operators' positions, the number of operators' equipments, 
and the number of pairs of cords and plugs. The number of jacks. 



224 AM ERIC AX TELEPHOXE PRACTICE. 

however, varies approximately as the square of the number of lines 
where the average number of lines served by one operator is the 
same. 

Since three operators occupy each section in a multiple switch- 
board and since the size of the section is such that one operator 
standing in the middle of the section can conveniently reach any 
of the jacks in that section alone, it follows that the middle operator 
at each section is the only one that can reach all over that particular 
section. The operator at the right-hand end of the section cannot, 
without undue exertion and without discommoding the operator in 
the middle position, reach to the extreme left-hand end of the sec- 
tion, and the same is true of the operator at the left, with respect to 
the right-hand end of the section. It is fortunate that this is not 
necessary. While the operator at the right-hand end of the section 
cannot reach the left-hand third of the same section, she can reach 
the left-hand third of the section at her immediate right ; and as this 
is an exact duplicate, so far as the multiple jacks are concerned, of 
the left-hand third of her own section, it serves her purpose equally 
well. The same is true of the operator at the left-hand position of 
the section. The operator at the left-hand position of the section 
uses the multiple jacks of the right-hand one-third of the section 
at her left, and of the left-hand two-thirds of her own section. The 
operator at the center of the section uses the multiple jacks of her 
own section only. The operator at the right of the section uses 
the multiple jacks of the right-hand two-thirds of her own, and the 
left-hand one-third of the section at her right. 

Unless special arrangements were provided the operator at each 
end position of the switch-board would have within her reach only 
two-thirds of the section ; thus the operator at the right-hand end 
position of the switch-board would have no left-hand third of a sec- 
tion to reach. There are two solutions of this difficulty. One is to 
place no operator at the end positions, equipping its panels, how- 
ever, with multiple jacks, as usual. This is the solution that is 
adopted by most of the independent manufacturing companies of 
this country. The Bell companies, however, as a rule, place an 
extra third section on the end of the multiple board, this section 
containing the multiple jacks necessary to complete the full quota 
of jacks necessarv for the reach of the operator at that end of the 
board. These two solutions amount to practically the same thing, 
although the latter requires an odd size section at the end of the 
board while the former does not. 



CHAPTER XIV. 
THE MAGNETO MULTIPLE SWITCH-BOARD. 

All the early multiple switch-boards were adapted to magneto 
signaling, the subscriber sending a call by turning the crank of the 
magneto generator. The signal receiving device at the central office, 
responsive to the current thus generated, was always some form 
of electromagnetically operated drop or target, several types of 
which have already been discussed. 

The first of these boards were adapted to use on grounded lines. 
The line circuit passed first through the spring of the jack of the 
first section, thence through the back contact of that jack to the 
spring of the jack located in the second section, and so through a 
jack in every section to the last, after which it passed through the 
coil of the annunciator to ground. Separate contact rings or 



rrr i 1 1 1 1 I T 1 1 TT 

FIG. 202.— GROUNDED LINE CIRCUIT FOR SERIES-MULTIPLE BOARD. 

thimbles were used in each jack for the purpose of giving a proper 
busy test. The test rings belonging to a line were wired together 
and so arranged that whenever a connection was made with that 
line all of the test contacts would be connected to a battery asso- 
ciated with the cord circuit used in making the connection. By this 
means an operator attempting to make another connection with a 
line already connected to at another section would receive a click 
in her head receiver. 

The circuit of such a line extending through the jacks on three 
sections is shown in Fig. 202. A cord circuit for association with 
such a line is shown in Fig. 203. This is a single-conductor cord 
circuit, carrying the usual listening key, L, two ringing keys. R and 
R'\ and a double-coil clearing-out drop, C 0, in scries with the cord 
strands. The operator's telephone circuit, including a test battery, 
T, of one or two cells, was bridged to ground from the point oi 
junction between the coils of the clearing-out drop, 

15 225 



226 



AMERICAN TELEPHONE PRACTICE. 



It will be seen that normally the circuit of the line extends through 
the contacts of the jack on each section to earth through the an- 
nunciator. A subscriber could therefore signal the central office 
at any time. The operator at whose position this annunciator was 
placed would respond by inserting the answering plug into the 
answering jack corresponding to the annunciator, which act would 
establish connection with the line and at the same time cut off the 
circuit of the drop by opening the contact between the spring 
and its anvil. The operator could then by means of her listening 
key ascertain the number of the subscriber called for, and before 
completing the connection would test his line by touching the 
calling plug to the test ring of the multiple jack at her section. 




^B 



CO DROP 



LI 



J_ 




T T 



FIG. 203.-CORD CIRCUIT FOR SERIES-MULTIPLE BOARD. 



at the same time keeping her listening key closed. If his line was 
free its test thimbles would have no ground connection, and there 
would be no flow of current through the operator's telephone set. 
She would therefore complete the connection by inserting the call- 
ing plug into the jack tested and ringing up the called subscriber 
by depressing the ringing key associated with the plug used. If, 
however, the line called for were already switched at another sec- 
tion, its test rings would all be connected to earth, and therefore a 
flow of current would take place on testing which would cause a 
click in the telephone of the operator making the test. The path 
of this current flow would be from the test battery, T, through the 
operator's telephone to the test rings of the line, and to ground 
through the telephone of one talking subscriber or both. 



MAGNETO MULTIPLE SWITCH-BOARD. 



22/ 



It was not long before metallic circuit lines became a necessity, 
particularly in multiple-board work, and after a long period of evo- 
lution the line circuit shown in Fig. 204 became the standard of 
the American Bell Telephone Company. This figure shows three 
lines passing through three separate sections in the multiple board. 
One side of each line, for instance of line 1, passes to all the con- 
tact rings, b, of the jacks belonging to that line. It then passes to 
one line terminal of the line drop, d f . The other side of the line 
passes to the spring, a, of the jack belonging to that line on section 
1, this spring resting against the anvil, c, to which a wire is con- 
nected running to the spring, a', of the jack belonging to that line 




FIG. 



204.— SIMPLIFIED DIAGRAM SERIES-MULTIPLE BOARD FOR 
METALLIC CIRCUITS. 



on the second section. So on the connection is made to all 
of the jacks belonging to this line, a wire leading from the anvil 
of the last jack to the other terminal of the annunciator. Lines 2 
and 3 pass successively through the sections in the same manner. 
This figure also shows the cord circuit stripped of details, the ar- 
rangement shown being best adapted to give an idea of the method 
of testing. 

When a subscriber operates his generator, the current passes over 
the line wire through all of the contacts, a and c, in series, through 
the drop-coil, and back over the other side of the line. When an 
operator inserts a plug into a jack, the spring, (7. is lifted from con- 
tact with the anvil, c, by the tip of the ping. The sleeve of the plug 



228 AMERICAN TELEFHOXE PRACTICE. 

makes connection with the test ring, b, and thus the tip and sleeve 
strands of the plug are connected, respectively, into the metallic 
circuit of the line, while the circuit through the drop is cut off at 
the anvil, c. 

The operator's telephone, T. may be then bridged across the cord 
circuit in order to enable the operator to converse with the sub- 
scriber who has called. Means for connecting the operator's tele- 
phone in the circuit in this manner are not shown in Fig. 204, the 
details of the cord circuit being described later in connection with 
another figure. This telephone in Fig. 204 is shown connected in 
a ground branch from the tip side of the cord circuit, in order to 
better illustrate the principles of testing in this system. 

The sleeve strand of each cord circuit is grounded through a 
battery, B. and in order that this ground may not produce serious 
effects in unbalancing or crossing the circuit of two connected lines, 
an impedance coil. /, is placed in this circuit. Whenever any plug 
is inserted into a jack, one side of the test-battery. B, is thrown 
on to all of the test-rings, b. of the line to which that jack belongs. 
If now an operator at another board desires to make a connection 
with that line she touches the tip of her answering plug to the test- 
ring, b, of that line. This will connect the test-ring, b, to ground 
through her telephone. T, and a click will be heard, due to the pas- 
sage of the current from battery, B. The operator will therefore 
know that that line is busy, and will refrain from making the con- 
nection. 

In Fig. 204 the three lines have their drops located at section 3. 
It must be remembered that other lines would pass through jacks 
on the various sections in a similar manner, but would have their 
drops located on sections 1 or 2. The operator at any section will 
of course, answer calls on lines terminating or having drops on 
her section only, but she may be required to connect one of these 
lines to any other line in the exchange by means of the multiple 
jack. 

The details of the cord circuit for this system are shown in Fig. 
205. K and K' are ringing keys for connecting the calling gene- 
rator with either of the plugs P or P' . The circuit between the 
plugs is normally maintained continuous, through the tip and sleeve 
strands, as can be readily seen. \\ nen the listening key. K" , is de- 
pressed, the condenser. C, is looped into the tip strand, and at the 
same time the operator's telephone circuit is bridged between the 
tip and the sleeve strand. A point in the operator's telephone cir- 



MAGNETO MULTIPLE SWITCH-BOARD. 



229 



cuit between the receiver and the secondary winding of the induc- 
tion coil is grounded so as to make the receiver available for test- 
ing. The test is made when the key, K" , is depressed, the test cir- 
cuit then being from the tip of the plug, P', through the tip strand 
to the right-hand spring of the key, and through its anvil and the 
receiver coil to the ground. The condenser, C, is for the purpose 
of preventing disturbances in the line with which the plug, P, is con- 
nected from giving a false busy test. 

The line circuit and cord circuit shown in Figs. 204 and 205 were 
at one time standard with the American Bell Telephone companies, 
the most notable case of their use being in the old Cortlandt Street 
exchange in New York. This was at the time the largest multiple 
switch-board in existence. While the old switch-board has been torn 
out to be substituted by one of modern construction, it formed a 




FIG. 205.— CORD CIRCUIT FOR METALLIC CIRCUIT SERIES- 
ULTIPLE BOARD. 



landmark in switch-board development, and therefore the actual cir- 
cuits employed in it may be of interest. There were both grounded 
and metallic circuit lines used in this exchange, the wiring of each 
line, as well as that of the cord circuit, being shown in Fief; 206, 
in which the line at the left of the figure is a metallic circuit, and 
that on the right a grounded circuit. There were in all 44 sections 
to this switch-board, the jacks in all of which, except the 44th. wore 
provided with a tip spring normally resting against a contact point, 
the circuit being broken on the insertion oi the plug. This jack is 
diagrammatically shown in detail, a, of Fig. 206. The jacks at the 
44th section were provided with a double break — that is. they were 
adapted to break both sides of the line instead of onlv one. The ar- 



230 AMERICAN TELEPHONE PRACTICE. 

rangement of springs in the jacks at this section is shown in detail. 
b i in Fig. 206. 

Section No. 44 was the first section to which the lines passed after 
entering the office, and this section was placed in the long- 
distance operating room on the floor below the other sections. The 
object of making a double-contact jack in this section was in order 
to completely cut off all connection with the main switch-board be- 
yond the 44th section when a connection was made between a local 
line and some long-distance line. 

The operator's cord circuit, as w r ill be seen, was like that shown in 
Fig. 205. The condenser was individual to each operator's position 
rather than individual to each cord. The purpose of this condenser 
was, as has been stated, to prevent a false "busy'' test. With the 
condenser not present it is evident that the tip of the calling plug 
when used in making a test would receive potential from the test 
battery over the circuit from ground, the test battery and 500-ohm 
retardation coil to the sleeve side of the cord, thence through the 
circuit of the line with which the answering plug was connected to 
the sub-station, and back to the tip side of the cord circuit and to the 
tip of the calling plug. This would render possible the flow of 
current from the tip of the plug used in testing to the sleeve of the 
line being tested whether that line was busy or not. It may be 
stated, therefore, more briefly, that the object of the condenser was 
to prevent the flow of current from the test battery to the tip of the 
calling plug through that portion of the circuit connected with the 
answering plug. A 

It is evident that when a metallic circuit line was connected with 
a grounded circuit line the talking circuit would be traced from 
ground at the subscriber's station on the grounded circuit line, 
through the line circuit to the central office, thence to the tip side 
of the metallic circuit line and back over the sleeve side of this line 
to the sleeve side of the cord circuit and thence from the sleeve con- 
tacts of the jacks on the grounded circuit line to ground at the cen- 
tral office. The presence of the 600-ohm non-inductive resistance 
in the ground connection at the central office on the groundedTine 
was to prevent the "dead grounding" of the test rings so as to keep 
their potential above that of the earth when a connection was made 
with the line so that the line would give the proper "busy" test while 
so connected. It is evident that if the test contacts of the grounded 
circuit lines had been dead grounded the potential of the test rings 
would have remained the same as that of the earth, regardless of 




4}|»|l|l|hfiS- 



232 AMERICAN TELEPHONE PRACTICE. 

whether the line was switched for use or not. Moreover, this con- 
dition would have short-circuited the test battery. 

The operation of the test is very simple. The test battery being 
permanently connected to the sleeve side of the cord circuit serves 
to raise the potential of all the test rings of the line as soon as, and 
as long as, any plug is inserted into a jack. When not connected 
the test rings are supposedly at the same potential as that of the 
earth. Therefore when an operator tests with the tip of a calling 
plug no current will pass through the circuit traced from the tip of 
her calling plug through her telephone receiver to ground in case 
the line is not busy, because there is no source of current in the 
circuit. If, however, the line is busy, the difference of potential 
between the test ring and the tip of the testing plug will be noticed 
by a click of the receiver. The circuit over which this testing cur- 
rent passes may be traced to ground at the central office through the 
test battery, through the sleeve of the testing plug, thence to the 
sleeve contact of the jack at which the test is being made, through 
the tip of the testing plug to ground through the operator's receiver. 

The test battery used in connection with the Cortlandt Street Ex- 
change consisted of 3 cells of a storage battery giving approximately 
6 volts. 

The multiple switch-board circuits so far shown are those of what 
are termed the series-multiple board, the name series being derived, 
of course, from the manner in which the line circuit passes through 
the contact springs and anvils of the multiple jacks. This system, 
although once widely used, is subject to grave defects and may now 
be considered obsolete. The series-multiple board has been replaced 
by another form of multiple board known as the "bridging" or 
'"branch terminal multiple." In the series-multiple an open circuit 
may be caused in any one of the jacks by a particle of dust or other 
foreign insulating matter becoming lodged between the line-spring 
and its anvil, or by virtue of one of the springs becoming weak and 
failing to bear upon its anvil. The liability to open circuits, there- 
fore, is very great, especially in large exchanges. 

Another serious objection to the series board is that when a plug 
is inserted into a jack, one side of the line is cut off at the anvil of 
that jack, but the test side is not cut off, and is continuous through 
the drop of that line and back to the anvil of the jack which is 
plugged. This, in a large exchange, means that to one side of the 
line is attached an open branch, perhaps several hundred feet long, 



MAGNETO MULTIPLE SWITCH-BOARD. 



233 



and containing the drop coil. This destroys to a certain extent the 
balance of the line, and is liable to produce cross-talk. 

The branch-terminal system was designed to remedy the defects 
inherent in the series system, and possesses many advantages over 
it, chief among which are the facts that when a connection is made 
with any line the balance of that line is in nowise affected, and that 
the liability of open contacts in the jacks, which is such a serious 
defect in the series system, does not exist. The branch-terminal 
system, moreover, lends itself more readily to the use of self-restor- 
ing drops, as will be described later. 

In Fig. 207 is represented the highest type of the branch-terminal 




FIG. 207.— SIMPLIFIED DIAGRAM OF BRANCH TERMINAL 
MULTIPLE BOARD. 

system, as applied to magneto signaling. This is sometimes called 
the three-wire system on account of the fact that three wires for 
each line extend throughout the multiple. In this figure three dis- 
tinct line circuits are shown passing through three sections of board. 
The wires k and k' of each line have branch wires leading off to a 
jack on each board; the branches from wire k leading to the contact- 
thimbles f, and the branches from wire k f leading to the short 
springs c, in the same jacks. Bridged across the two wires oi each 
line is the line coil n' of the individual annunciator belonging to 
that line. This coil is high wound in order that it may lie left per- 
manently bridged across the line without materially affecting the 
efficiency of the system in talking*. 



234 AMERICAN TELEPHONE PRACTICE. 

A third wire, t, passes through the board in parallel with each 
line. From this wire branch wires are run to the test-thimble, g, 
and to the spring, b', in each jack belonging to that line. The test 
wire, t, after passing through all of the boards, runs through a low- 
resistance coil, ;r, on the drop of the line to which that particular 
test wire belongs, and then passes to ground through a battery, o, 
common to all test wires. These test wires are represented by dotted 
lines in the figure in order to distinguish them more readily from 
the line wires. The remaining spring, b. in each jack is permanently 
connected to a ground wire, G, common to all of the jacks. 

Each plug in this system is provided with two contacts, h and /, 
which form terminals respectively of the sleeve and tip strands of 
the cord circuit. The tip, h, registers with the spring, c, when the 
plug is inserted into the jack (see Fig. 208), and the sleeve, /, regis- 
sters with thimble, f. A conducting ring, i, entirely insulated from 
all other portions of the plug, registers with the springs, b, and 
b', in the jack, and connects them together electrically. 




FIG. 20S.— THREE-CONDUCTOR PLUG AND TACK. 

Three general results are accomplished by the insertion of a plug 
into a jack. The tip and sleeve strands of the cord circuit are con- 
nected respectively with the sides, k' and k, of the line, thus 
continuing the line circuit to the cord circuit. The connecting of 
springs, b and b', by the ring, i, completes the circuit of the battery, 
o, through the restoring coil, n 2 , of the annunciator, to the ground 
wire, G, and thus allows current from this battery to energize the 
coil, n 2 , and restore the shutter of the annunciator. Lastly, the con- 
necting of springs, b and b\ by the ring, i, connects the test-thimble, 
g, to ground by a short circuit, so that when an operator at any other 
board touches the test-thimble of that line with the tip of her plug, 
a signal will be given denoting the line as busy. 

In the normal or idle condition of a line, the test-ring, g, is elec- 
trified to a difference of potential from the earth by the battery, 0, 
which finds circuit through the restoring coil, ;? 2 , of the annunciator 
of that line to all the different test-rings, g, belonging to that line 
at all of the sections of the board. If when the line is in that con- 
dition the tip of the test-plug, which is grounded through the oper- 



MAGNETO MULTIPLE SWITCH-BOARD. 



235 



ator's receiver and the same battery, be applied to the test-ring, no 
current will flow through the receiver because both the tip and the 




test-ring are at the same potential. Silence will therefore indicate 
a free line. 

When, however, the line has been put into use by the insertion 
of a plug into the spring-jack thereof, the springs, b and b', are con- 



236 AMERICAN TELEPHOXE PRACTICE. 

nected by the contact-ring, i, carried on the plug, whereby all the 
test-thimbles, g, belonging to that line are connected directly to 
earth through a short circuit, and therefore no difference of potential 
exists between them and the earth. Thus, when a test is made on 
a spring-jack of that line there will be a flow of current through the 
operator's receiver to ground, and a click will be the result. 

Fig. 207 is stripped of all unnecessary detail in order to enable 
the general underlying principles to be more readily grasped. In 
Fig. 209 the same system is shown more in detail as to circuits, con- 
nections, and apparatus. Fig. 207 will give the reader a better un- 
derstanding of how the jacks are grouped into sections, and of the 
relative location of the parts, while Fig. 209 will enable a better 
study of the circuits. 

In this figure two subscribers, 1 and 2, are shown connected by 
line wires, k and k', with the exchange. Jacks / and /', at sections 
1 and 2 of the board, are shown in connection with the line leading 
from station 1. Jacks I 2 and / 3 are shown connected at the same 
sections with the line leading from station 2. Across the line lead- 
ing from station 1 is bridged the line coil, n', of the annunciator, 
this annunciator being placed at section 2 of the board. The line 
coil of the annunciator of line 2 is similarly bridged across the two 
sides of the line, and is placed at section 1 of the board. A little 
study will show that the circuit of the line wires and the test wires 
are the same in Figs. 207 and 209, although represented in an en- 
tirelv different manner. Like letters correspond to like parts in 
these two figures. 

Two pairs of connecting plugs and their accessory appliances 
are shown complete, one at each section of the switch-board. The 
tips of the two plugs of a pair are connected together by one of the 
conductors, q, of the flexible cord, and the sleeves, 7, are likewise 
connected by the conductor, q\ of the same cord. Included in cir- 
cuit between the two plugs of a pair are two calling keys, r and r' , 
each adapted to disconnect both contact-pieces of one of the plugs 
from those of the other, and to connect them to the anvils, s and s', 
which form the terminals of the calling generator, P. G. 

A listening key. **, is provided for each cord circuit, having con- 
tact points or anvils connected with the conductors, q and q\ as 
shown, and having its contact-spring, u' and ?r. connected with the 
terminals of the operator's telephone, w. When the plunger of the 
listening key. u, is allowed to rise, the operator's telephone is con- 
nected in a bridge across the two sides of the cord circuit, as is 



MAGNETO MULTIPLE SWITCH-BOARD. 237 

shown at section i. A wire is connected from the middle point of 
the coil of the operator's telephone receiver to ground through the 
battery, o, so that when a test is made of any line, as was described 
above, a circuit will be completed from the contact-thimble, g, of 
the jack through the tip strand, q, of the cord circuit, and thence" 
through one-half of the operator's receiver coil to the ground. As 
this wire leads from the center part of the operator's receiver coil, 
it may be left connected permanently, as it does not desroy the bal- 
ance of the line. 

A clearing-out annunciator, x, similar in construction to the line 
annunciator, has its high-resistance coil, x' , bridged permanently 
across the two sides of each cord circuit. The restoring coil, x 2 , 
is connected in a normally open local circuit, including the battery, 
o, and terminating in the ground on one side, and in a spring, u 3 , on 
the other. This spring, ir, is arranged in conjunction with the lis- 
tening key in such a manner that when the key is raised the spring 
will make contact with a grounded anvil, d. Thus, whenever the 
operator listens in on any cord circuit she at the same time restores 
the clearing-out drop if it happens to be down. 

In order to give a clearer understanding of the system so far de- 
scribed, it will be well to follow the operation in connecting one 
subscriber with another. Suppose Subscriber I desires connection 
with Subscriber 2. He operates his generator, 1, and the current 
therefore passes over the line wires, k k', and through the coil, n', 
at section 2 of the board. The operator, noticing this signal, inserts 
plug, p, into jack, /'. This completes the circuit from ground, 
through battery, 0, coil, n 2 , of the line annunciator, thence to spring. 
b\ through the ring, i, on the plug to spring, b, and to ground. The 
front armature of the annunciator is therefore attracted and the 
drop restored. 

The operator then connects her telephone across the cord circuit 
by raising the key, u, and communicates with Subscriber Xo. 1, in 
order to ascertain his wishes. Having found that he desires a con- 
nection with Subscriber No. 2, she takes up plug, />'. of the same 
pair and tests to find out whether line No. 2 is connected to at some 
other board. If it is busy a current will pass from battery, 0, 
through one-half of the coil of her receiver, and one part of the 
secondary to the spring, u', in the listening key. From this spring 
it passes to the tip strand, q, of the cord circuit, to the tip. //. oi the 
testing plug. As the test-thimble to which the plug is applied is 
grounded by the insertion of a plug at another board, the current 



238 AMERICAN TELEPHONE PRACTICE. 

will pass through it to ground. This will produce a click, which will 
indicate to her that the line is busy, and she will not complete the 
connection called for. If, however, she finds the line to be free 
she thrusts the plug entirely into the jack, in which position it is 
shown in the figure, and depresses the key, r', in order to throw 
current from generator, P. G, upon the line of Subscriber No. 2. 

The two subscribers are now connected for conversation. When 
either rings off the current passes through the coil, x', bridged 
across the cord circuit, and actuates the clearing-out drop. The 
operator, noticing this, again listens in, by raising the key, u, in order 
to find out whether they are through talking, or whether one of 
them desires another connection. The act of listening in closes 
spring, a 3 , against anvil, d, and thus restores the shutter of the 
clearing-out drop. If the subscribers have finished talking, the 
plugs are removed and placed in their normal resting place. 

If, while subscribers 1 and 2 were connected together at section 
2, as above described, someone at section 1 had desired connection 
with, say, line No. 2, the operator at section 1, in applying the tip 
of her plug, p 3 , to the test-thimble, g. as shown, would receive a 
click in her receiver for the reason, as pointed out above, that con- 
tact, g, is connected to the ground by a short circuit by the plug 
inserted in jack. / 3 . No difference of potential would, therefore, 
exist between thimble, g, and the ground, and hence a current from 
the battery, 0, would pass through the telephone of the operator 
making the test. 

Success in practical telephone working can be attained only by 
the greatest attention to matters of detail. Nowhere is this fact 
better illustrated than in the design of the various parts which go 
to make up a multiple board. In the construction of large boards 
of this type, the possible capacity of the board is limited by the 
number of spring- jacks that can be placed within the reach of a 
single operator. It is evident, therefore, that space must be econo- 
mized to the last degree, and yet the jacks must be substantial, in 
order to resist the wear and tear of years of service ; must be made 
easily removable so as to be accessible for repairs ; must perform 
their electrical functions with absolute certainty, and at the same 
time be so arranged as to facilitate the orderly and systematic con- 
nection of the wires leading from the line cables. 

Moreover, when it is considered that single multiple boards often 
contain many hundred thousand spring-jacks, it may be easily 
realized that the cost of producing these jacks must be seriously 



MAGNETO MULTIPLE SWITCH-BOARD. 



239 



considered. It is well to state here, however, that any economy in 
the construction of a switch-board that will tend to decrease its dura- 
bility and reliability of action is poor economy indeed. 

As an illustration of the highest development attained in spring- 
jack construction during the period when the magneto branch- 
terminal multiple board was in greatest use, the spring jack used in 
that board will be described. The jacks were mounted in strips of 
twenty and so arranged that each strip could be removed from the 



** a 5 \ 


a 5 \ J 














K> 




f\ 


n 


P 


Ft 


*7 








7 








b' 


A 






'Of 

e 


iA 


1° 

SIS 


of 


X 

m 


of I 



n 




board by the removal of two screws which bound it firmly to the 
framework. Figs. 210 to 213, inclusive, show the details of the 
construction of one of these jack strips. 

The hard rubber strip, a, forms the framework for each strip of 
twenty jacks. The projections at its ends provide for attachment 
to the switch-board. In this strip are milled, on its upper side, the 
transverse grooves, a' a', and on its lower side similar grooves, a 2 a 2 ; 
these being best seen in the right-hand portion of Fig. 213. 

Holes are drilled from the front of the strip, one perforation to 
9 



a 9 



a 




FTC. 211.— FRONT AND REAR VIEWS OF MULTIPLE TACK STRIP. 

each pair of grooves, each hole having its axis centrally located and 
parallel with respect to the grooves. A small portion of the hard 
rubber is removed from between the grooves so as to leave a rectan- 
gular opening, <7°, as shown in Figs. 212 and 213, through the strip 
connecting the two grooves at those ends which are nearer the front 
of the jack. In the grooves, a', upon the upper surface of the rub- 
ber strip are mounted springs b and c. The spring, b, is the longer 
of the two, so that its curved extremity is presented close to the end 



240 



AM ERIC AX TELEPHONE PRACTICE. 



of the perforation through the front portion of the strip, a. The 
springs are insulated from each other by a strip or tongue, d, of hard 
rubber, thin and flexible enough not to prevent the flection of the 
two springs. In the under groove, a 2 , is mounted another spring b' 
similar to spring b, and of equal length. 

The three springs, b, c, and b', are firmly secured to the strip a, 
by a bolt e, passing through them and the body of the strip, a. The 
bolt is insulated from the springs, b', and c, by rubber washings and 
bushings. In the perforations, a 3 , in front of the strip, are inserted 
short tubes, /, of brass. Each tube or thimble, f, is provided with 
a shoulder, which bears against a corresponding ledge in the per- 
foration, a 2 , so as to prevent the tube from being thrust toward the 
rear of the jack by the insertion of the plugs. The thimble, /, is 
provided with an extension, f, to afford electrical connection with it 
from the rear of the jack. This strip, f, extends through an oblique 



S<Sj |StS)||9f3| ©rS] |©|S> i©|S\ rSpi prBjg 



a 



M 




UCil 



FIG. 212.— BOTTOM VIEW OF MULTIPLE JACK STRIP. 



duct, f 2 — shown in dotted lines in Fig. 212 — and thence through a 
transverse slot or saw-cut, a 4 , to the rear of the strip. 

In front of the thimbles, /, in the perforations, a 3 , are placed the 
test-rings, very short tubes of brass, g. These are forced into place 
against other ledges in the perforation. The ring, g, is also pro- 
vided with an extension, g', projecting to the rear of the strip of 
spring-jacks. These extensions, g, are of wire and pass through 
another duct, g 2 , in the front portion of the strip, a, into a saw-cut, 
a 5 , thence to the rear of the strip, where they are connected with the 
spring, b. 

The springs in these jacks are of hard German silver, which has 
been found the most desirable material for this and similar purposes. 

It will thus be seen that each of these jacks had five contacts, 
three springs, and two sleeves or thimbles. Each thimble contact 
was formed of two pieces, the thimble proper, and its rearwardly 
extending wire. These jacks, on account of their complexity, could 
not be made small enough to be mounted on less than half-inch 



MAGNETO MULTIPLE SWITCH-BOARD. 



241 



centers, and therefore the limit of the capacity of multiple boards 
of this type was approximately 6000 lines. One of the greatest 
problems in switch-board engineering has been the production of 
systems that would allow of a spring-jack of much simpler construc- 
tion. 

A section of one of the plugs used with these jacks is shown in 



1 



r 



h ^f 



FIG. 213.— DETAILS OF JACK. 



-frr 



Fig. 214. The tip, h, of brass is secured by the rod, h', to the block, 
h 2 , also of brass. Insulated from the tip portion by a rubber bush- 
ing is the sleeve contact, If, of the plug, which projects rearwardly 
and forms the main body of the plug. Over this portion is slipped 
a shell, If, of hard rubber or fiber, which forms a handle for the 
plug. Between the tip and sleeve, and insulated from each, is the 
ring, h 7 , which, as was described before, is for short-circuiting the 
springs, b and b', when the plug is inserted in the jack. 

Screw connectors, If and If, form convenient terminals for attach- 
ing the strands of the cord to the tip and sleeve, respectively, of the 
plug. These connectors are always readily accessible for inspec- 
tion or repair by the removal of the sleeve, h 6 . 

Typical of the highest development of the magneto mutiple 
switch-board is the board used at Paris, France, installed bv the 




FIG. 214.— DETAILS OF PLUG. 

Western Electric Company. This is not only the largest magneto 
multiple board ever installed, but is one of the largest single multiple 
boards of any kind in the world. A front view of a portion of this 
switch-board is shown in Fig. 215, this view comprising a little over 
one section. Immediately above the key-shelf are the answering 
10 



242 



AMERICAX TELEPHONE PRACTICE. 



jacks, above which are the trunk jacks in which lines leading to 
other exchanges terminate. Above these, and occupying by far the 
greater portion of the jack space, are found the multiple jacks. 




FIG. 213.— FRONT VIEW OF PARIS SWITCH-BOARD SECTION. 



Above the multiple jacks are shown the line and clearing-out sig- 
nals. Those at the top of each space are the line drops, the clear- 
ing-out drops occupying the two lower rows. All drops are of the 
electrically self-restoring type shown in Fig. 178. The drop spaces 



MAGNETO MULTIPLE SWITCH-BOARD. 



24:j 



at the top of the board are divided in accordance with the oper- 
ators' positions, there being three such positions to each section. 
The key shelves are also divided in the same manner. The face 




FIG. 216.-GENERAL VIEW OF PARIS SWITCHBOARD. 



of the board, however, which includes the jack, is divided into seven 
panels for each section. The horizontal white linos extending 
through the face of the board are dividing lines between each bank 



244 AMERICAX TELEPHOXE PRACTICE. 

of ioo jacks, there being 5 strips of 20 each in each panel between 
each two horizontal white lines. Each jack panel, except the sev- 
enth, has thirteen banks of jacks, the seventh having twelve. There 
are thus ninety banks, or 9000 multiple jacks in each section. It 
is thought that practice has shown that this section is too large, and 
that with the size of jack used this number (9000) cannot be well 
handled. 

The disadvantage of having the answering jack separate from 
the calling signal is particularly marked in the case of this board. 
The operator has first to look up to the top of the section and fix 
the number of the signal displayed, in her mind, and then look down 
to pick out the corresponding answering jack in which to insert 
the plug. In such large multiple boards the placing of the line 
signal so far above the normal range of vision of the operator rend- 
ers necessary an unnatural position of the head of the operator, 
with a consequent strain upon her nerves. 

In Fig. 216 is shown a view of one-half of this great Paris switch- 
board, the sections being arranged in two rows, of which one only 
is here shown. It has an ultimate capacity of 9100 lines. 



CHAPTER XV. 

TRANSFER SYSTEMS. 

No person of intelligence can visit a modern central office con- 
taining a large multiple switch-board, without being deeply im- 
pressed by a certain magnificence of the equipment, as a whole, as 
well as by the perfection of the system in its minutest detail. If he 
is conversant with telephone matters, he must also be impressed 
by the fact that while the multiple switch-board gives the subscriber 
what he needs — quick, reliable service — it gives it only at a great 
initial cost to the operating company. There is no question 
that at the present stage of telephonic development the multiple- 
board systems represent the highest type of central-office equipment, 
and while one form or another of them is in use in nearly all the 
really large exchanges the world over, there are a few notable ex- 
ceptions. On account of the apparent success of some of these, and 
of the expense necessarily entailed in the installation of large mul- 
tiple boards, it is not to be wondered at that telephone men are 
constantly seeking a system which, while it may in some degree 
embody the plan of multiple jacks, will mainly depend for its action 
on other ideas. 

In the multiple switch-board one part of the cost of installation 
increases as the the square of the number of the subscribers. This 
is clearly an objection which increases in seriousness as the ex- 
change grows. 

The enormous multiplying of jacks in the multiple system is for 
one purpose — to enable each operator to have within her reach a 
terminal of every line in the exchange, to the end that she may be 
able herself to complete any connection called for over any one of 
the lines under her immediate supervision: that is, that she may be 
able to answer any call arising at her section, and, without requiring 
the aid of any other operator, make the connection called for. In 
other words, in the switch-board for 24,000 lines, cited on page ? , 
1,944,000 spring-jacks would be used, instead of onlv 24,000, in 
order to accomplish this result. This appears to be a great cost 
to pay for such a simple result, but up to the present date practical 
experience has proved that the end justifies the means. 

245 



246 AMERICAN TELEPHONE PRACTICE. 

When two or more operators instead of one must handle the 
connections between two subscribers, as must always be the case 
where the multiple system is not used, the liability of error is about 
doubled, and the fact that the attention of both is simultaneously 
required on the same connection, necessarily slows down the ser- 
vice. The saving in cost over the multiple system is not as great as 
it appears at first, for in order to make the co-operation of the oper- 
ators as effective as possible, a complicated system of automatic sig- 
nals must be installed, which adds greatly to the complexity of the 
apparatus and circuits. It is a fact, however, that many medium- 
sized exchanges, and one large one, are being operated with appar- 
ent success without the use of multiple boards. 

Systems depending for their operation on the transfer of a con- 
nection from one portion of a board to another are termed transfer 
systems, and one of the most successful of these is the so-called 
"express system" of Messrs. Sabin & Hampton of San Francisco. 
This system has been used for several years in San Francisco, and 
has long been cited as having demonstrated its capability of hand- 
ling with success an exchange having over 6000 lines, and several 
times that many subscribers. 

The system is so radically different from anything so far described 
that its consideration in detail should be a matter of interest. One 
striking feature in it is that no magneto generators are used at the 
subscriber's station, the calling being accomplished automatically by 
the raising of the subscriber's receiver from its hook. The doing 
away of the magneto, however, is not an essential feature of this ex- 
press system, but is one of the advantages incident to its use. 

Briefly stated, the underlying ideas of the express system are 
as follows: The boards are divided into two classes, termed for 
convenience "A" and "B." Similarly, the operators at the respect- 
ive boards are termed "A" operators and "B" operators. There 
is but one line jack for each line in the exchange, and these are 
divided into groups of one hundred each and are placed only at 
the "B" board. At the "A" boards, which are entirely removed 
from the "B" boards (they may even be in another office), are placed 
plugs which form the terminals of trunk lines leading from the 
various "B" boards; and also jacks forming the terminals of other 
trunk lines leading to the "B" boards. 

The trunk lines terminating in plugs at the "A" boards also ter- 
minate in plugs at the "B" boards. These are termed "A" trunk 
lines. The trunks terminating in jacks on the "A" boards terminate 



TRANSFER SYSTEMS. 247 

in plugs on the "B" boards, and are termed "B" trunk lines. When 
a call is received it attracts the attention of one of the "B" operators 
by displaying an annunciator in the ordinary manner. The "B" 
operator at whose board the call is received pays no further atten- 
tion to it than to insert one of the plugs of an "A" trunk into the 
jack of that line, thus transferring the call to an "A" operator, who 
answers it with a listening key in the ordinary manner. No means 
whatever are provided for a "B" operator to listen in on a sub- 
scriber's circuit, this duty being confined solely to the "A" operators. 

The "A" operator, having learned that the subscriber calling de- 
sires to be connected with a certain other subscriber, conveys this 
information, by means of a special order wire, to the "B" operator 
at whose board the called-for subscriber's line terminates. The 
"B" operator then tells the "A" operator what "B" trunk line to 
use, and the "A" operator then inserts the plug of the "A" trunk 
line used into the jack of the "B" trunk line thus designated. This 
brings the connection as far as the board of the second "B" oper- 
ator; that is, the "B" operator at whose board the called-for sub- 
scriber's line terminates. This operator, in order to complete the 
connection, simply inserts the plug of the "B" trunk used into the 
jack of the called-for subscriber's line, and presses a ringing key 
in order to call that subscriber. 

It will be seen that the connection has really been handled by 
three different operators, but that the first of these operators does 
no more than to insert a plug into a jack, giving the matter no 
further attention. 

All signaling between the subscribers and the operators and be- 
tween the various operators, whether it be for establishing or clear- 
ing out a connection, is entirely automatic, and therefore not de- 
pendent upon the volition of the parties concerned. 

Fig. 217 shows the arrangement of the circuits at the subscribers' 
stations, and also the arrangement of the spring-jacks and annun- 
ciators on the "B" boards. One side of the line wire at the sub- 
scriber's station is normally grounded through the polarized ringer. 
R. This means that calling a subscriber from the central office 
must be accomplished over one limb of the line wire and ground, 
instead of over a metallic circuit, as in case of talking and other 
signaling in this system. The other circuits and apparatus at the 
subscriber's station are of the ordinary arrangement and type, the 
only difference being that the magneto-generator is omitted entirely 
The line wires of each subscriber terminate in two springs, a and b, of 



248 



AMERICAX TELEPHOXE PRACTICE. 



their spring-jack, /. These springs normally rest on two anvils, c 
and d, one of which connects through an annunciator, c, w T ith a 
heavy wire leading to one pole of the calling battery, and the other 
of which leads to a similar wire connecting with the other pole of 
this battery. 

This annunciator, c, has a shutter which is simply lifted by the 
attraction of the armature, and again dropped into its normal posi- 




SUBSCF*IBER"S LIMES. 






CALLIH5 BKTTEJVr 

FIG. 217.-SUBSCRIBERS' CIRCUITS-EXPRESS SYSTEM. 

tion when the armature is released. It is, therefore, the simplest 
type of self-restoring drop. The circuit of the call-battery is nor- 
mally open only at the subscriber's station. It is automatically 
closed through the receiver and secondary winding of the induction 
coil at the subscriber's station whenever the subscriber removes his 
receiver from its hook. This allows enough current from the 
calling battery to pass through the drop, c, to raise its shutter, and 
thus attract the attention of the "B" operator at that board. 

The shutter remains raised until the operator inserts the plug of 
one of the "A" trunk lines in order to transfer the call to the "A" 
operator. The insertion of this plug, however, lifts the springs, a 
and b, from the anvils, c and d, thus cutting off the battery and 
allowing the shutter of the annunciator, e, to drop to its normal 
position, and the "B" operator therefore pays no more attention 
to it. 



TRANSFER SYSTEMS. 249 

A single battery is made to serve for actuating the signals of 
every line in the exchange, no matter how great their number may 
be. Storage cells are used for this purpose, ten cells being con- 
nected in series so as to give a pressure of about twenty volts. It 
is said that the average flow of current from this battery is about 
one and one-half amperes, and never exceeds two amperes, in the 
San Francisco exchange of approximately 6000 subscribers. It will 
be thus seen that the cost of current supply for these batteries is 
trifling. 

Another good feature of this arrangement is that, should a "B" 
operator by mistake withdraw a plug from a jack before a sub- 
scriber has finished talking, — that is, before he has hung up his 
receiver, — she will be at once notified of her mistake by the display 
of a signal belonging to that line. 

In Fig. 218 is shown a simplified diagram of the express system. 
At the bottom and top of this figure are show T n the subscribers' lines, 
leading in each case from the subscriber's telephone apparatus to 
the drop and jack at the central office. This part of the apparatus 
is the same as that shown in, Fig. 217, but in this figure the details 
of the local circuit at the subscribers' stations have been omitted for 
*;he sake of clearness. 

The jacks and drops belonging to these lines are, as has already 
been stated, stationed at the "B" boards of the exchange. The 
subscriber's indicator battery is represented at S* I B and the indi- 
cators at /. Leading from the section of the "B" board shown at 
the top of the figure is a trunk line leading to a plug on the second 
section on the "A" boards. It will be noticed that this trunk line 
terminates in a plug at each end, and is termed the "A" trunk. An 
intermediate jack and plug are shown in the circuit on this "A" 
trunk, but these at present need not be considered. Suffice it to say 
that the plug at the "B" board is connected by a metallic circuit to 
the plug at the second section of the "A" board. Leading from a 
certain jack on the second section of the "A" board is a trunk line 
extending to a plug on another section of the "B" boards. This 
is termed a "B" trunk. Only one "A" trunk and one "B" trunk 
are shown, but it must be remembered that a number of "A" trunks 
lead from each of the "B" boards to the "A" boards, and that from 
the "A" boards" a number of "B" trunks lead back to each of the 
"B" boards. 

When a subscriber, as for instance the one shown at the top of 
the figure, removes his receiver from its hook, his indicator, /. is 



250 



AMERICAN TELEPHONE PRACTICE. 



displaced automatically, and the operator at the particular "B" board 
at which this indicator is located extends the circuit of his line to 
one of the "A" boards over an "A" trunk. This she does by insert- 
ing the plug of an "A" trunk into the jack of the calling subscriber. 




The operator at the "A" board, having learned the desire of the 
calling subscriber, extends the circuit to that subscriber's line by 
means of the "B" trunk, still further on to the particular "B" board 
at which the jack of the called subscriber is placed. This the 



TRANSFER SYSTEMS. 251 

"A" operator does by inserting the plug of the "A" trunk used into 
the jack of a "B" trunk at her board. The "B" operator at whose 
board the calling subscriber's jack is placed then completes the con- 
nection between the two subscribers by inserting the plug of the 
"B" trunk used into the jack of the calling subscriber's line. The 
two subscribers' lines are shown connected in Fig. 218 by the process 
and over the circuits just described. 

In order to facilitate matters it is evidently necessary that a most 
complete set of signals must be provided between operators. The 
first in this series of signaling operations is to notify the "A" 
operator that her attention is desired on a certain "A" trunk. This 
must always occur just after the "B" operator has inserted one of 
the "A" trunk plugs into the jack of the calling subscriber's line. 
It will be noticed in Fig. 218 that the relay, R, operating signal lamp, 
L, is bridged across the tip and sleeve strands of the "A" trunk 
circuit, and this bridge may be traced from the tip strand through 
the balance coil, B C, thence through the clearing-out indicator bat- 
tery, C B, to the battery wire, thence through the coil of the relay 
at the "A" board, and thence to the sleeve strand of the "A" trunk. 
Remembering that the calling subscriber at the top of the figure has 
removed his receiver from its hook, then the insertion of the plug 
of the "A" trunk will restore the line drop, /, and at the same time 
will close the circuit from the clearing-out indicator battery, COB, 
and the relay, R, through the subscriber's line and telephone instru- 
ment. This will operate the relay and cause it to close the circuit 
of the signal lamp, L, thus calling the attention of the "A" operator 
to the fact that an unanswered call is upon the trunk line to which 
that lamp belongs. 

The apparatus of the "A" operator is shown more clearly in Fig. 
219. The sleeve and tip strands of the "A" trunk are shown at the 
extreme left of this figure. When the armature of the relay, R, is 
attracted, as described above, the circuit from the local battery and 
white lamp is completed at the point, b, of the relay. It will be 
noticed that this local circuit extends through two of the springs. 
e and f, normally closed, on the listening key of the "A" operator. 
and also through a pair of contacts, / and r, held closed by the weight 
of the plug of the "A" trunk in its socket. The white lamp will 
therefore remain lighted until one of the following three things 
happens: until the operator listens in. which causes the local circuit 
to break at the listening key; or until she removes the plug of that 
trunk line from its socket, which would break the local circuit at 



AMERICAN TELEPHONE PRACTICE. 



the point, c; or until the calling subscriber hangs up his receiver, 
which would cause the relay to let go of its armature, and thus break 
the circuit at point b. The white lamp remains lighted, therefore, 
as long as the call on its "A" trunk is unattended to. 

The first act of the "A" operator on seeing this light is to throw 
her lever corresponding to that light into its horizontal position, 
thus connecting her telephone to the terminals of the "A" trunk 
in the usual manner. This enables the "A" operator to communi- 
cate with the calling subscriber in the ordinarv manner. It should 




FIG. 219.— TABLE OF "A" BOARD. 

be noted that these keys on the "A" operator's board are the only 
means afforded to any operators for communicating with sub- 
scribers. The operation of this key breaks the circuit of the white 
lamp at the points, c f. As soon as the listening key is thrown again 
into its normal position the white lamp is again lighted, thus calling 
the operator's attention to the plug to be used in making the con- 
nection. 

This precaution is a wise one, for before making the next move 
in the connection the "A" operator must communicate with the 



TRANSFER SYSTEMS. 253 

"B" operator at whose board the called-for subscriber's line ter- 
minates. The "A" operator does this by depressing her order-wire 
key, shown in Fig. 218, but omitted from Fig. 219, which act con- 
nects the "A" operator's telephone directly with the telephone set 
of the outgoing "B" operator. The "A'' operator then tells the "B" 
operator the number of the line with which connection is desired, 
and the "B" operator in return tells the "A" operator the number 
of the trunk line she is to use in making the connection. 

It will be noticed that the trunk jacks of the "B" trunks are in 
reality arranged on the plan of the multiple board! This is for the 
purpose of placing within the reach of every "A" operator a jack 
belonging to every "B" trunk line. No test system, however, is 
required on these jacks, as an "A" operator always first learns from 
an outgoing "B" operator which "B" trunk to use, and of course 
a "B" operator would never designate any trunk which was already 
in use, or "busy." 

We have now carried the connection, or extended the circuit of 
the calling subscriber's line, as far as the trunk line plug at the 
outgoing "B" board. The outgoing "B" operator then completes 
the connection by inserting this plug into the jack of the called sub- 
scriber. The "B" operator then depresses her ringing key, shown 
in simplified form in Fig. 218, which sends calling current from the 
generator, G, over the sleeve strand of the plug cord thence to line 
and to ground through the polarized call-bell at the subscriber's 
station. 

The next feature to consider is that of the automatic clearing-out 
signals. As the connection between the two subscribers is made 
by three operators, it is evident that three distinct clearing-out sig- 
nals should be given, one at each of the boards of the operators 
who help establish the connection. Turning again to Fig. 219. it 
will be seen that the raising of the "A" trunk plug from its socket 
changed the circuit of the local battery from the white lamp to the 
red lamp, by moving the selecting lever, /, from the point, r. to the 
point, d. Remembering now that as long as the calling subscriber's 
receiver is off its hook, the circuit from the clearing indicator bat- 
tery is closed through the relay, R, at the "A" board, thus attracting 
its armature ; as soon, therefore, as the calling subscriber finishes 
his conversation, he hangs up his receiver, and thereby breaks the 
circuit through the relay at the "A" board, thus closing the circuit 
through the red lamp. This lamp will therefore be lighted as a 
notification to the "A" operator that disconnection on that trunk 



254 



AMERICAN TELEPHOXE PRACTICE. 



is desired. The replacing of the plug in its socket opens the circuit 
of the red lamp at the point, d, thus extinguishing the lamp. This 
apparatus at the "A" board is very ingenious, and deserves special 
attention. It should be noticed that should an operator by mistake 
remove one of the "A" plugs, and replace it in its socket before the 
subscriber connected had hung up his receiver, the white lamp 



ra 



■J m ■ 



ifcfcWj 



1L 



tin 



To C.O.B. 



B> TFOJHK 



/in' 



^^3 



71 



I 



tm, 



\ 



4r@r) 






SS4 




FIG. 220.-TABLE OF "B" BOARD. 



would be relighted, thus calling the attention of the operator to 
the error. 

The clearing-out signal is given to the incoming "B" operator 
in much the same way as that on the "A" board, the clearing in- 
dicator, or relay, on the "A" trunk of the "B" board being wired 
in multiple with the relay on the "A" board. 

The clearing-out signal on the outgoing "B" board is accom- 
plished by much more complicated means, and will be explained 



TRANSFER SYSTEMS. 255 

by reference to Fig. 220. In this figure the ringing key, f, is shown 
in more detail than in Fig. 218. The "B" trunk line jack on the 
"A" board is represented by a. In the normal position of the key 
the two strands of the "B"' trunk are connected to the tip and 
sleeve of the corresponding plug on the outgoing "B" board. 
When, however, the key is depressed, the sleeve strand of the cord 
is connected with the calling generator, the other terminal of which 
is grounded. When the operator depresses this ringing key, a 
secondary pair of contacts, i i 1 , are closed, thus actuating the lower 
magnet, /, of the compound relay, and causing it to attract its arma- 
ture, I 1 . When the operator allows the key, f, to rise, the armature 
/, falls back, but is caught by the hook, m 1 , of the upper coil, m, 
of the relay. The hook, m 1 , and the tip of the armature are pla- 
tinum-pointed, and their contact causes the signal lamp, w, to be 
lighted. This lamp remains burning until the called subscriber 
takes his telephone off the hook, which act closes the circuit through 
the combined clearing-out relay and signal, c, in exactly the same 
manner as the relays on the incoming "A" trunk line were oper- 
ated. The operation of this r.elay therefore closes a circuit at the 
points, c 2 c 3 , through the upper magnet, m, causing it to raise the 
hook, m 1 , and allow the armature, /\ to drop back. This extinguishes 
the lamp, n, and shows the operator that the subscriber has re- 
sponded. The armature, c 1 , of the relay, c, remains attracted until 
the called subscriber hangs up his receiver, which de-energizes the 
magnet, c, and allows the signal carried by the armature to resume 
its normal position. This is the clearing-out signal for the out- 
going "B" operator, and she accordingly pulls out the plug. 

To review the action of the indicators at the outgoing "B" 
board, the releasing of the key for transmitting a calling signal to 
the subscriber lights the lamp, n, and shows the operator that this 
part of her work has been attended to. The response of the sub- 
scriber is indicated by the going out of the lamp, and bv the raising 
of the signal, k. The clearing-out signal is given by the lowering 
of the signal, k. 

We have now traced through the operation of all the signals be- 
tween the subscribers and the operators, and between the operators 
themselves, which were necessary to establish a connection between 
two subscribers ; and also the subsequent signals between the sub- 
scribers and the operators, indicating that a disconnection is desired. 
The striking feature of all this elaborate system of signaling is that 
each signal is automatically given without the volition of the opera- 



256 AMERICAN TELEPHONE PRACTICE. 

tor or subscriber, inasmuch as it is brought about by some action 
necessary in the actual connection or disconnection. 

In order to reduce the work of the operators to the last degree, 
two phonographs are placed in connection with the exchange, one 
of which is constantly and politely repeating the sentence, "Busy. 
Please call again," while the other repeats with equal regularity, 
"Subscriber called for does not reply." Each of these phonographs 
speaks to a transmitter arranged in connection with an induction 
coil and battery in the ordinary manner. The terminals of the sec- 
ondary of the induction coil of the "busy" phonograph terminate in 
a jack on each section of the "A" boards. In like manner the "does 
not reply" phonograph is connected with a jack on each section of 
the "B" boards. When, therefore, an "A" operator learns that a 
line called for is busy, she inserts the plug of the "A" trunk to 
which the calling subscriber is connected into the phonograph jack, 
and the familiar but disappointing message, "Busy. Please call 
again," is automatically conveyed to the calling subscriber. 

In a similar manner the outgoing "B" operator may inform the 
calling subscriber that the subscriber called for does not respond. 

The writer is indebted to an able article by Mr. George P. Low, 
in the Electrical Journal, for much information concerning this very 
interesting exchange system. 

Perhaps the best argument that can be given for the multiple 
board, as against various transfer systems, is that this system is soon 
to be replaced in San Francisco by the modern multiple switch-board 
system. It was tried with modifications by the Bell company in Chi- 
cago. The result of this Chicago trial, conducted with great care 
in two offices in actual service, was to establish the great value of 
automatic signals but not to establish that the express form of 
divided switch-board was as good and as economical as the multiple 
form. This led to the design of the central battery multiple system. 

Fig. 221 is a schematic representation of the system used several 
years ago, in the larger exchanges installed by the Western Tele- 
phone Construction Company, of Chicago. This system proved 
fairly successful for exchanges up to fifteen hundred subscribers, 
although with a larger number certain difficulties were met in the 
disposal of the transfer plugs. In this figure I, 2, 3, 4, etc., represent 
different sections of the board, each section having one hundred com- 
bined drops and jacks of the type shown in Figs. 180 and 181, to- 
gether with a complete operator's equipment. One answering plug, 
A, together with one calling plug, B, is shown at each section. These 



TRANSFER SYSTEMS. 



257 



are connected together in pairs through clearing-out drops, O, 
by ordinary flexible cords which contain the necessary switching ap- 
paratus for enabling an operator to listen and to ring out over either 
cord as desired. Connected with each of these sets of plugs is a 
trunk line to which is connected at every third section a transfer 
plug, C, as shown. Thus, a pair of plugs, A and B, shown at section 
I, is connected by means of a trunk line to a transfer plug at section 
4, another at section 7, another at section 10, and so on. A careful 
consideration of this figure will show that the same is true for each 
pair of plugs, A and B, at the other boards. Fig. 222 shows in 



m 



12 



$ \0 w m \b\ iwf M M tel M V k M k . 




FIG. 221.— GENERAL SCHEME OF WESTERN TRANSFER SYSTEM, 



greater detail one pair of plugs, A and B, connected by a trunk line 
to the several transfer plugs according to this system. The plugs. 
A and B, are in this case at section 4, while the transfer plugs. 
C, C, C, are at sections I, 7, and 10, or in other words, at every third 
section on each side of section 4. By throwing the lever, L, to the 
left, its two springs are connected with an operators circuit through 
the secondary winding of the induction coil, as shown, while when 
the lever is thrown to the right the terminals of the generator, G. 
are connected with the plug circuit. 

The system can now be more readily understood by describing its 
operation. If a subscriber whose line terminates in section 4 calls 

17 



258 



AMERICAN TELEPHONE PRACTICE. 



up, the call is answered by the operator at that board by inserting 
one of her plugs, A, the insertion of this plug restoring the shutter 
mechanically. The operator then throws the lever, L, to the left, 
connecting her telephone set, E, with the line of the subscriber 
calling. Having learned that a connection with, say, subscriber No. 
iooi is desired, the operator at 4 depresses the key, K, which con- 
nects her telephone set with an order-wire circuit, /, terminating in 
the telephone set of the operator at section 10. The operator at sec- 
tion 4 is thus enabled to communicate with the operator at section 
10 over this circuit, and the former informs the latter of the num- 
ber desired, and of the particular transfer plug, C, she is to use in 
making this connection. The operator at section 10 then takes up 











x 1 1. 1 z 


3 1 4 1 5 


6 | 7 | 8 


9 | 10 | X 




FIG. 222.-TRANSFER AND ORDER-WIRE CIRCUITS. 



the plug, C, designated and inserts it into the jack of the called sub- 
scriber, the operator at section 4 meanwhile holding the lever, L, 
of the particular plugs used, into the ringing position. As soon 
as the connection is completed at section 10 the first operator is 
informed of the fact by the operation of a buzzer placed in the 
cord circuit, so that she knows that the signal has been properly 
transmitted to the line of the subscriber 1001. After the conversa- 
tion is completed, one or both subscribers ring off, which throws 
the clearing-out drop, O, and informs the operator at section 4 
that a disconnection is desired. She therefore removes her answer- 



TRANSFER SYSTEMS. 259 

ing plug and places it in the socket, informing the operator at sec- 
tion io to do likewise. 

It will be seen that, in addition to the transfer lines extending 
from the answering and calling plugs at each board to transfer 
plugs at each third board therefrom, a system of order-wire circuits 
is also provided, each circuit terminating in an operator's set at 
one board and connected with push-butttons at every third section 
therefrom, so that an operator is enabled to communicate only 
with those operators located at every third section from her own 
board. This peculiar arrangement serves several advantageous 
purposes, among which is the reduction of plugs necessary for the 
successful operation of the board, and also the reduction of the 
number of operators talking over any one instruction circuit. It 
moreover enables any operator to reach, by means of her own call- 
ing plug or transfer plug handled by another operator, any portion 
of the board. It has been shown how a connection is made between 
section 4 and some section at which one of the transfer plugs of that 
section is located. If, however, the subscriber on section 4 had 
called for a subscriber at section 9 the operator at 4 would have 
signaled the operator at 10, who would then have completed the 
connection, using transfer plug, C, with her left hand. If the 
called-for subscriber had been upon section 8, operator No. 4 would 
have signaled No. 7, who would have used a plug, C, at her section 
with her right hand to complete the connection. Ten pairs of 
calling and answering plugs are furnished for each section of 100 
drops, each pair being connected by trunk line with transfer plugs 
distributed through the system as already described. 

A system of lamp signals for facilitating the work upon these 
boards was devised and used in many of the later exchanges. In 
this a white light is so arranged in connection with the night-alarm 
circuit as to be illuminated, upon each board, whenever a drop is 
thrown. A similar lamp in series with this is also arranged to be 
displayed on the chief operator's table, thus serving as a telltale to 
call the attention of the chief whenever a drop remains unattended 
on any section. A colored lamp is arranged in connection with 
each set of transfer plugs and controlled by normally open contact 
points in the plug seats of the transfer plugs and normallv closed 
contact points in the plug seats of the answering points. Two 
lamps are arranged in series in each circuit, one at the set of trans- 
fer plugs to which it belongs and the other at the set of answering 
plugs with which these transfer plugs communicate. Whenever an 



260 AMERICAN TELEPHONE PRACTICE. 

operator raises an answering plug in order to establish a connection 
the lamp circuit is opened at that point by the operation of the con- 
tacts in the plug seat. When another operator removes the transfer 
plug to complete the connection this same lamp circuit is closed at 
that point by the operation of the contacts in the transfer plug seat. 
When a clearing-out signal is received, and the operator removes the 
answering plug to take down the connection, the lamp circuit is 
closed at its only open point, which lights the lamp in front of each 
operator. This shows the transfer operator that a disconnection 
is desired, and also shows the answering operator that the discon- 
nection has not yet been made. The cycle of events is completed 
when the transfer operator removes the transfer plug and replaces 
it in its seat, which act opens the lamp circuit at that point, thus 
putting out both lamps. 

The switch-board of the Delmarvia Telephone Company at Wil- 
mington, Del., is shown in Fig. 223. This board embodies all the 
features mentioned above and is representative of the highest de- 
velopment attained by the "multiple plug" transfer system. 

What is known as the Cook-Beach transfer system was once used 
to a considerable extent by some of the Bell and Independent ex- 
changes of medium size. The subscribers' lines terminate in drops 
and jacks on the various sections of the board, no multiple con- 
nection whatever being used between them. A set of transfer jacks 
is also provided on each section, these jacks being connected by 
trunk lines extending to transfer plugs located at the several sec- 
tions. When a call is received at any section, the operator answers 
it by inserting one of her answering plugs into the corresponding 
jack. Having learned the number of the subscriber called for, 
she inserts the corresponding connecting plug into the transfer jack 
connected by a trunk line with a plug at the board where the line 
of the subscriber called for terminates. She then communicates 
by order wire with the operator at that board, who picks up the 
transfer plug designated and inserts it into the jack of the called 
subscriber. The connection between any two subscribers is thus 
made complete by the use of three plugs. This style of transfer 
system has proven its adaptability to service in magneto systems 
of medium size, where the number of calls per subscriber per day 
was not excessive. 

The most notable success made in the handling of transfer sys- 
tems in the Eastern portion of the United States has been made by 
the City Telephone Company of Grand Rapids, Mich. The system 



191- 



w. ' / 

L 




2G1 



262 



AMERICAN TELEPHOXE PRACTICE. 



used in the exchange of this company is a modification of the Cook- 
Beach system, being shown in some detail in Fig. 224. 

It may be said that as applied to manual switch-boards the trans- 
fer system has been, and is being, gradually relegated to the past. 




FIG. 224.— GRAND RAPIDS TRANSFER BOARD. 



Of the two largest transfer systems the one at San Francisco, al- 
ready described, is about to be replaced by a multiple system. The 
next largest, that of the Citizens' Telephone Company at Grand 
Rapids, Mich., giving service to over 5000 lines, is now replaced by 
an automatic switch-board of the Strowger type. 



TRANSFER SYSTEMS. 263 

It is not to be inferred from this, however, that the transfer sys- 
tem has no present field of usefulness. Many small exchanges 
which at the start require no multiple board or transfer system, soon 
grow to such an extent as to render necessary increased switch- 
board capacity. The switch-board thus enlarged often becomes too 
large for a single set of operators to reach o,ver its entire face, and 
therefore either the adoption of the multiple or the transfer system 
becomes necessary. To adopt the former would be to throw away 
the present switch-board equipment and install a multiple board — too 
expensive an operation for many small companies to afford. Small 
magneto switch-boards usually lend themselves readily to the addi- 
tion of the transfer system, which may thus make the old equipment 
serve its enlarged field of usefulness. 

In such transfer systems great speed is not, as a rule, required, 
and therefore as good a form of transfer circuit as any is one ter- 
minating in a jack at each end. Two of these transfer circuits are 
usually led from each ioo-line section to every non-adjacent section. 




(ft =*= (ft 



7TJ 



IP W 

FIG. 225.— TRANSFER CIRCUIT FOR SMALL BOARDS. 

One of these circuits is shown in Fig. 225. The jack at each end 
comprises a tip and sleeve contact, t and s, respectively, connected 
by the two sides of the talking current. Moving with the tip spring 
of each jack is a spring, 1, which normally rests against a contact, 2, 
but when a plug is inserted, breaks the contact and moves into 
contact with the spring, 3. Two signal lamps, L L, one at each 
section between which the transfer circuit extends, are arranged in 
series between the springs, 1, of the two jacks. The springs, 2, are 
connected together and to one pole of a battery, while the springs. 3, 
are connected to ground or a common return, as is also the remain- 
ing pole of the battery. With these connections, it will be scon that 
if a plug is inserted into a jack at either end, the other jack remain- 
ing normal, both lamps will be lighted. If both jacks are vacant. 
or both plugged, the lamps will be extinguished. The operation 
of such a transfer system is as follows: When a call is received at 
any section of the board, the operator plugs into the corresponding 



264 AMERICAN TELEPHONE PRACTICE. 

jack and answers the call in the ordinary way. If the subscriber 
desired is within the reach of this operator, that is, on her own sec- 
tion or that at her right or left, this operator completes the connec- 
tion with her own pair of plugs and cords in the ordinary manner. If, 
however, the call for the subscriber is on some section not within, 
the reach of the answering operator, this operator inserts a calling 
plug into the jack of the transfer line leading to the section of the 
subscriber called for, and notifies the operator at that section by 
order wire of the connection required. 

The insertion of the plug by the first operator lights the lamp 
at each end of the transfer line. This serves as a guide to the second 
operator as to the transfer line she is to use. This operator there- 
fore inserts the answering plug of one of her regular pairs into the 
transfer jack, thus putting out both lamps and indicating to the first 
operator that the connection has been made. The second operator 
then completes the connection by inserting the calling plug of the 
pair used in the jack of the called subscriber, and rings in the ordi- 
nary manner. When a clearing-out signal is received the clearing- 
out drops of the cord circuit at both sections fall. 



CHAPTER XVI. 

SYSTEMS OF TRANSMISSION IN COMMON BATTERY 
EXCHANGES. 

It is an obvious disadvantage to have two separate sources of 
current at every subscriber's station in an exchange ; and it is only 
natural that early efforts were made to centralize the transmitter 
batteries and the calling current generators. By bringing about 
such a centralization of the sources of energy many desirable re- 
sults are attained. The idle capital represented by the local bat- 
teries and the calling generators is done away with — no small con- 
sideration in large exchanges, because the magneto-generator is 
in itself the most expensive part of an ordinary telephone set. The 
labor and expense of visiting or inspecting the subscribers' appa- 
ratus is greatly reduced; that neccessary to repair and renew bat- 
teries, together with the expense of material for such renewal, be- 
ing rendered nil. The subscribers' instruments are made neater, 
simpler, and more compact. The electrical efficiency of the plant 
is greatly increased by having a few large units in operation prac- 
tically all of the time, instead of a great number of small units in 
operation but a small portion of the time. No freezing of the local 
batteries occurs ; there is no spilling of the acids or other chemicals, 
and no corrosion of the various parts by fumes therefrom. 

The above advantages are those which at once become apparent 
when the subject is considered from the standpoint of maintenance 
and first cost. There are other and no less important advantages, 
however, of the common battery system, due to the fact that these 
systems lend themselves to automatic signaling to a degree not 
attainable in the old magneto system. The fact that a large source 
of electrical energy stands ready at the central office to be drawn on 
at any time by the subscriber, makes possible the conveying of a 
definite signal to the central office when the subscriber removes his 
receiver from its hook, and of another when he again hangs it up 
after use. As a result, the signals which the subscriber uncon- 
sciously sends to the central office are much more dependable and 
intelligible to the operator than those he sent, or was supposed to 
send, under the old regime by turning the crank of his magneto gen- 

265 



266 AMERICAN TELEPHONE PRACTICE. 

erator. This latter thought suggests the fact that by taking away 
from the subscriber all voluntary acts as far as possible, another 
advantage is gained; all work is performed by skilled operators 
trained for the purpose, rather than leaving, it to be done partially 
by the public, made up of people of varying intelligence, and at 
best, unskilled. The subscriber thus gets with common battery 
systems service with less effort, and as a direct result of this bet- 
ter service. 

Besides the advantages mentioned of reducing the work on the 
part of the subscriber, the labor of the operator has been so greatly 
lessened by modern methods of signaling in common battery sys- 
tems as to enable her to handle with success an average of about 
twice as many subscribers as with the old system. 

A general preliminary discussion of common battery systems 
naturally resolves itself into two divisions : first, the means by 
which speech transmission is effected; second, means by which the 
various signals necessary to the proper working of the system are 



JT 



+£-. 

^n 



FIG. 226.— SERIES COMMON BATTERY TRANSMISSION— GROUNDED. 

transmitted'. This chapter will deal mainly with the first of these 
divisions. 

Some of these advantages of the common battery system were 
appreciated by telephone men long ago, and many attempts were 
made at an early date to realize them in practice. The first efforts 
involved a return to first principles, doing away with the induc- 
tion coil and placing the transmitters and receivers of two con- 
nected subscribers directly in series with the circuit of the two con- 
nected lines. In one of these, made in 1881, by George L. Anders, 
the transmitter batteries were placed in a loop used to connect the 
circuit of two lines. In this the switch-board was of the old cross- 
bar type, and, while it used no cord circuits, the batteries were 
placed in series in the connecting wire corresponding to the cord 
circuit in later exchanges. 

This general method, as applied to a board having plugs and flex- 
ible cords is illustrated in Fig. 226, where 1 and 2 represent two sub- 
scribers' stations, connected at the central office, C, by a pair of 



COMMON BATTERY TRANSMISSION. 



267 



plugs, P and P', having a battery, B, included in circuit between 
them. The transmitter and receiver of each subscriber's station 
are placed' in series on the line wire, and each transmitter when oper- 
ated serves to vary the resistance of the entire circuit formed by the 
two connected lines, and to thereby vary the strength of the current 



n 

-h 



H 



C 



X 



n 

■+ 



FIG. 227.— SERIES COMMON BATTERY TRANSMISSION— METALLIC. 

flowing from the battery, B } producing corresponding effects in the 
receivers. 

In Fig. 22J the same principle of operation is applied to metallic- 
circuit lines, two of which are shown connected at the central office, 
C» by the pair of metallic-circuit plugs, P and P' . In both of these 
cases, in which the battery is included in the cord' circuit in series 
with the combined ciruit of the two lines, the use of a separate bat- 
tery for each cord circuit is, under ordinary circumstances, necessary. 
This is always true of the grounded system shown in Fig. 226, and is 
also true of the metallic-circuit system shown in Fig. 227, unless 
the battery, B, is made to have a very low internal resistance. This 
fact was pointed out by Mr. Anthony C. White, who, in 1890, stated 



1 



T>< 



7r\ 



("5 W 



— |i|i|i|h \a J 



J] 
-+ 



+■ 



*( 



T 



FIG. 228.-SINGLE BATTERY SERIES SYSTEM. 

that it was possible to supply all of the cord circuits from a single 
battery by connecting them in the manner shown in Fig. 228. This 
involves the bunching together of one side of each of the cord cir- 
cuits, the battery supplying current in multiple to the various pairs 
of lines in use at one time. This figure shows four stations, 1, 2, 
3 and 4, connected in pairs by two cord circuits and pairs of plugs. 



268 AMERICAX TELEPHOXE PRACTICE. 

Fluctuations set up by the transmitter in the line of subscriber, L, 
will circulate in the combined circuit of the lines of subscribers. I 
and 2. Similar fluctuations set up by the transmitter at 3 will flow 
through the circuit of the lines, 3 and 4. The battery. B. is com- 
mon to both of these line circuits, and if the resistance from the 
point, a. to the point a\ through the batten- is considerable in 
amount, a part of the fluctuations flowing in the circuit of sub- 
scribers 1 and 2 will be shunted by this resistance through the com- 
bined circuits of the subscribers 3 and 4. If, however, the resistance 
from the point, a. to the point. a\ is made extremely small, practically 
all of the current changes will flow through the batten- instead of be- 
ing shunted around through the circuit of the subscribers 3 and 4, 
owing to the comparatively high resistance and impedance of that 
circuit, with its included instruments. The desired reduction in the 
resistance between the points, a and a', may be accomplished by mak- 
ing the battery, B. of extremely low resistance, and by shortening 

J C a, Z 

+ — r>\^j>^ — ¥ 

FIG. 229.— STONE COMMON BATTERY ARRANGEMENT. 

the wire, a a\ which is common to all of the circuits. The former 
result is accomplished by using a storage battery of rather large 
capacity, and the latter by joining the various cord circuits indi- 
vidually to the bus-bar of the battery, so as to practically eliminate 
all resistance in the wire, a a'. 

In all arrangements where the transmitters are in series with the 
battery and the combined circuit of the two connected subscribers' 
lines, the transmitter is required to vary the resistance of the entire 
circuit connecting the two stations, and. in the case of long lines, this 
may be very high. Such systems are faulty, in that the resistance 
variation caused by the transmitter may be extremely small as 
compared with the total circuit resistance, and when this is the 
case the variations of current will, therefore, be small. [Moreover, on 
account of the high line resistance the steady current from the 
battery will be small unless a large number of cells are used. 

The common battery arrangement shown in Fig. 229 is one whicb 
has come into extensive use. and was designed by Mr. John S. 



COMMON BATTERY TRANSMISSION. 



269 



Stone in 1892. It eliminates to some extent the objection just men- 
tioned. 

In this figure 1 and 2 are, as before, two subscribers' stations 
connected by metallic-circuit lines, with the central office at C. 
The transmitter and receiver at each station are connected in series 
in the line circuit. The battery, B, however, is connected between 
the two sides of the cord circuit, terminating in the plugs, P and P'. 
On each side of the battery is placed an impedance coil, / and /', 
as shown. The action in this is as follows : The current from the 




FIG. 230.— STONE COMMON BATTERY ARRANGEMENT. 



positive pole of the battery, B, flows through the impedance coil, I, 
to the point, a, where it divides, a part passing through the receiver 
and transmitter of each of the subscribers' stations. The two parts 
of the current, after flowing back to the central office through the 
opposite sides of the lines, unite at the point, a', and flow through 
the impedance coil, /', to the negative pole of the battery. Inas- 
much as the impedance coils are of low ohmic resistance, they offer 
but little obstruction to the passage of this current. If, now, the 
transmitter at station 1 is caused to lower its resistance, the differ- 
ence of potential between the points, a and a' , will be lowered. This 
will result in a diminution in the current flowing in the line of sub- 
scriber 2. On the other hand, if the resistance of the transmitter 
is raised, the difference of potential between a and a' will be raised. 



270 AMERICAN TELEPHONE PRACTICE. 

thus causing a greater current to flow through the instrument of 
subscriber 2. Every fluctuation in the resistance of the transmitter, 
caused by sounds at either station, will thus cause corresponding 
fluctuations in the current flowing through the receiver at the other 
station, thus causing them to reproduce the sounds. The same bat- 
tery, B, is used to supply a large number of cord circuits, the ar- 
rangement being then, as shown in Fig. 230, each side of the vari- 
ous cord circuits being connected to the poles of the battery through 
impedance coils, as before. The fluctuations set up in the circuit of 
the two subscribers 1 and 2, while perfectly free to pass through 
these two particular lines, cannot find a path to any other lines, 
as, for instance, those of subscribers 3 and 4, without passing 
through the impedance coils, / and 1 /' and also P and P. 

An objection to the Stone system is that when a long line is con- 
nected to a short one, the short one, on account of its low resistance, 
takes a comparatively large current, thus causing a considerable 
drop in potential through the coils, / and /', and thus reducing the 
difference of potential between the points a and a'. As a result the 
long line gets little current, while the short line, which needs less, 
gets more. 

Early in 1892 Mr. Hammond V. Hayes devised the method of 
current supply to transmitter batteries shown in Fig. 231, this hav- 



J 



C 



2 VsOpM i 



FIG. 231.-HAYES COMMON BATTERY ARRANGEMENT. 

ing come into very extended use in the Bell companies, and it has 
formed the basis of some of the most successful common battery 
systems in the world. The apparatus at the subscribers' stations, 1 
and 2, may for present purposes be considered arranged in all of 
the preceding systems. At the central office, K K' are repeating 
coils, each having two windings, k and k'. The two windings of the 
coil, K, are connected together at the point, a, which is connected 
with one pole of the battery, B. The other ends of these two wind- 
ings are connected with the tip contacts of the plugs, P and P', as 
shown. In an exactly similar manner the two windings of the re- 
peating coil, K', are connected together at the point, a', which is 



COMMON BATTERY TRANSMISSION. 271 

connected with the other pole of the battery, B, the other two ends 
of these windings being connected with the sleeve contacts of the 
plugs, P and P'. By this arrangement the battery is included in a 
bridge conductor between the sides of the circuit formed by the two 
connected lines, and one limb of each line includes one of the wind- 
ings of one of the repeating coils. The current from the battery, L ', 
will, when the subscribers' receivers are removed from their hooks, 
divide at the point, a, and pass in multiple through the two wind- 
ings of the repeating coil, K, thence the two portions of the current 
will pass through the transmitter and receiver of the two sub- 
scribers' stations, respectively, and back to the repeating coil, K' , 
through the windings of which they pass to the negative terminal 
of the battery. 

Any changes in the current in either circuit, produced by the 
operation of one of the transmitters, will act inductively through 
the repeating coils upon the other circuit, causing corresponding 
fluctuations in current to flow through that circuit and actuate its 
receiver. Thus, when the subscriber at station I is transmitting, 
the windings, k k, will operate as primary windings of a trans- 
former, of which the secondary is formed by windings, k' k'. When 
the subscriber, i, is transmitting, this action is reversed, k' k' serv- 
ing, as primary windings, and k k as secondary windings. The 
transmitter at any station is compelled to vary the resistance of its 
own line circuit only, and in this way some of the advantages of 
a local circuit are gained. 

While two repeating coils, K and K', have been referred to in the 
foregoing description, they have a common core, which has therefore 
four windings. These are, under ordinary circumstances, all of the 
same number of turns. Such a coil is often referred to as a ''split re- 
peating coil," the windings, k k, together forming one side — say 
the primary — while the other two coils, k' k\ form the other side, or 
secondary. 

The system of supplying battery current to subscribers' trans- 
mitters, used by the Kellogg Company, has points in common with 
the system of Stone, although it differs from it materially. In the 
Stone system, as has already been pointed out, a serious disadvantage 
exists due to the fact that when a high resistance line is connected 
with a low resistance line the difference of potential at the terminals 
of the impedance coils will be greatly lowered by the presence of the 
short line, thus cutting down the amount of current that would be re- 
ceived by the long line. This must always be the case where two lines 



272 AMERICAN TELEPHONE PRACTICE. 

of unequal resistance draw their current supply from the battery 
through the same coils. This defect is clearly overcome in the Hayes 
system. 

In the Kellogg system each line draws its current through a 
separate pair of impedance coils, and in fact from a separate 
battery. The scheme of using two batteries for central battery 
work was proposed by the writer, and has been adopted as the uni- 
versal practice of the Kellogg Switch-board and Supply Company. 
The particular arrangement of circuits shown in Fig. 232, which is 

J £ * 



2 WIOTW I 



t ' ^ v f p? 1* \ J. 



FIG. 232.— KELLOGG TWO-BATTERY ARRANGEMENT. 

also representative of present Kellogg practice, was proposed by Mr. 
W. A. Taylor. In this plan, instead of depending upon direct con- 
ductivity between the two lines, as in the system of Stone, or upon 
the electro-magnetic induction between two sides of a repeating coil, 
as in the system of Hayes, the electro-static induction afforded be- 
tween the plates of two condensers is depended upon for transmitting 
the fluctuations in current from one line to the other. The using of 
two batteries instead of one has some practical advantages, while the 
presence of the condensers, instead of cutting down the talking 
efficiency, as might be inferred from casual observation, tends rather 
to improve it. In fact, experiments made with a circuit of this kind 
have shown that the transmission was better with the condensers con- 
nected, as shown, than it was when the condensers were short-cir- 
cuited. 

In all the systems so far described current has been supplied to the 
subscriber's talking apparatus over two limbs of the line in series. 
Mr. J. J. Carty is responsible for the broad idea of supplying current 
to the transmitter of the subscriber's station over the two sides of a 
metallic line in parallel, using the ground as return. This method, as 
worked out by Mr. Carty, has been improved upon by Mr. W. W. 
Dean, who has produced an extended series of inventions embodying 
this feature. One of them is shown stripped of details in Fig. 233, in 
which 1 and 2 are two subscribers' stations, and C the central office. 
/ is an impedance coil bridged across the two sides of the cord circuit 



COMMON BATTERY TRANSMISSION. 



273 



of the plugs, P and P'. The center point, a, of this coil is grounded 
through the common battery, B. The receivers at the subscribers' sta 
tions are connected serially with the secondary coil, s, of an induction 
coil in the metallic circuit formed by the two sides of the line wire. V 
is an impedance coil bridged between the two sides of the line circuit 
at each subscriber's station, the center point, f, of this coil being con- 
nected with one side of a primary circuit containing the transmitter, 
T, and the primary coil, p, of the induction coil. The opposite side 
of this primary circuit is grounded at the point, g. Current from the 
battery, B, flows to the center point, a, of the impedance coil, I, in the 
cord circuit ; thence through the two sides of this coil in multiple to 
the points, b and c, on the opposite sides of the cord circuit. From 
these points the current flows over the two line wires in multiple to 
the points, d and e, from which they flow through the two sides of 
the impedance coil, /', at the subscriber's station to the point, f, where 
they unite. The current then passes to the primary circuit, where it 
again divides, part passing through the transmitter, T, and part 
through the primary coil, p. It reunites at the point, g, and passes 
to the ground and back to the battery, B. 

Variations in the resistance of the transmitter at one of the stations 
cause more or less of the supply current to be shunted through the 
primary, p, of the induction coil, and these varying currents through 
the primary induce corresponding currents in the secondary, s, placed 
directly in the line circuit with the receiver. These currents flow 
over the metallic circuit formed by the two connected lines, and are 







FIG. 233.— DEAN COMMON BATTERY ARRANGEMENT. 

prevented from flowing through the bridge wires, d c and b c, bv the 
presence of the coils, /' and /, contained therein. In the modification 
of this scheme Mr. Dean uses a transmitter having two variable- 
resistance buttons, one of which decreases the resistance of its circuit, 
while the other increases the resistance of its circuit. One of these 
buttons is placed in each of the branches of a primary circuit, such as 
is shown at the subscriber's station in Fig. 233, each side oi the 
is 



274 



AMERICAN TELEPHONE PRACTICE. 



circuit also containing the primary of an induction coil. These are 
so arranged with respect to the secondary that an increase in current 
flowing through one of them produces the same effect on the second- 
ary as a decrease of current in the other, and, therefore, the effects 
produced by the tw r o variable-resistance buttons of the transmitter are 
added. 

The use of secondary batteries at the subscribers' stations supplied 
by some source of current, either at the central office or elsewhere, 
has received attention since the very early days of telephony. Storage 
batteries are, in many respects, peculiarly fitted for telephone work. 
Their extremely low internal resistance, and their ability to maintain 
a constant E. M. F. for a considerable period, are obvious advantages 
over the primary battery. Charles E. Buell, of Plainfield, X. J., was, 
in 1881, the pioneer in this line. He was followed by Stearns in 
1883, Dyer in 1888, and Dean, Stone, Scribner, McBerty and others, 
who have accomplished much in this line of work since 1893. The 
idea of Dyer in 1888 was to charge the storage battery from the ordi- 
nary lighting mains of a city, the battery then acting in a local cir- 
cuit containing the transmitter and primary of an induction coil, in 
the same manner as when a primary battery is used. 

In Fig. 234 is shown one of Stone's methods, which involves the 
use of an electrolytic, or secondary cell at each of the subscribers' 
stations. In this, advantage is taken of the fact that if, when a stor- 




234.— STORAGE CELL AT SUBSCRIBER'S STATION. 



age cell is entirely discharged, a charging current is sent through it, 
a considerable counter E. M. F. is set up by the cell. One terminal of 
the battery, B, at central is grounded, its other terminal being con- 
nected to the center point of a divided repeating coil, bridged across 
the cord circuit of the plugs, P P', after the manner of the Hayes sys- 
tem. Across the terminals of the line wires at each subscriber's sta- 
tion is connected the secondary, s, of an induction coil, the center point 
of which is grounded through a secondary cell, S. In circuit with this 
cell is the primary, p, of the induction coil, together with the trans- 
mitter. The current from the battery, B, passes in multiple over the 



COMMON BATTERY TRANSMISSION. 275 

two line wires, through the transmitter and secondary cell in multiple, 
and returns by ground. When the transmitter is operated variations 
in current in the local circuit at the sub-station are produced, and 
these act inductively on the line circuit containing the receivers by 
means of the induction coil. If the cell, S, is discharged the trans- 
mitter may be considered as acting solely by means of the battery, B, 
the counter E. M. F. of the electrolytic cell serving to divert a con- 
siderable portion of this current through the transmitter, and thereby 
accomplishing the same result as if the current originated in the cell, 
S, itself. If, however, the cell, S, is fully charged, then the trans- 
mitter may be considered as working upon the current generated by 
it, and would so work whether the battery, B, were in circuit or not. 
The fact that the secondary cell possesses practically no resistance 
and no inductance renders it especially advantageous for this work. 

The use of storage batteries or electrolytic cells at subscribers' sta- 
tions makes possible a full realization of the advantages of the induc- 
tion coil, but, of course, introduces the disadvantages of having fluid 
cells at points remote from the central office. They have been used in 
some cases with apparent success, but their advantages have not been 
sufficient to overcome the disadvantages mentioned. 

An electrolytic cell acts in a circuit very much in the same manner 
as a condenser, and systems have been devised in which condensers 
were used at the subscribers' stations in place of the cells, S, shown 
in Fig. 234. If we assume these cells to be replaced by condensers, 
the other arrangements of the circuit being left as shown, current 
from the battery, B, will pass over the two line wires in multiple, as 
before, and to ground through the transmitter, none of it being al- 
lowed to pass through the other branch of the primary circuit, by 
virtue of the condenser. When, however, the transmitter is caused 
to vary its resistance, the fluctuations in the current set up by it are 
readily transmitted through the condenser, which offers to them little 
impedance. These fluctuations therefore act inductively upon the 
secondary coil, s, of the induction coil, thus causing corresponding 
currents to flow in the metallic circuit in the ordinary manner. 

Instead of using a storage battery at the subscriber's station, Mr. 
Dean has proposed the use of a thermal generator, or thermopile, to 
produce the necessary current. As is well known, if the alternate 
junctions of a thermopile are heated, the others remaining cooler, an 
E. M. F. will be set up by the pile. An obvious way of supplying 
the heat is to wrap the juncture with high-resistance wire, which may 
be heated by the passage of a current through it. This Mr. Dean 



276 



AMERICAN TELEPHONE PRACTICE. 



does, and his simplest arrangement of circuits and apparatus is repre- 
sented in Fig. 235, in which the wires of a telephone line are shown 
leading to the central office of the telephone exchange. The telephone 
receiver, R, and the secondary, s, of the induction coil are placed in 
the line circuit, as in the instruments now in use. This line circuit is 
normally open, but is closed by the hook-switch when released from 




TO TELLCLF-HONE LINE! 



FIG. 235.— DEAX THERMOPILE. 



the weight of the receiver. The transmitter, 7\ the thermopile, C, and 
the primary, p, of the induction coil, are placed in series in the local 
circuit, which is permanently closed. 

The resistance coil, r, which is here shown in proximity to the 
thermopile, instead of being wrapped around it, is in a circuit in 
which is included a generator (either a dynamo or a battery). It is 
obvious that this generator may be placed at the central station, or 
that the current may be derived from the street mains of an ordinary 
electric li^ht circuit. The circuit through this coil, r, is normally 
broken at the hook-switch. When, however, the receiver is lifted 
this circuit is completed, and the coil, r, becoming heated, puts the 
thermopile into action. The thermopile therefore generates the cur- 
rent only as long as the telephone is in use, and the breaking of the 
primary circuit becomes unnecessary. The action of the apparatus 
in talking is precisely the same as if a chemical battery were used. 

Mr. Dean has worked out a system by which the current is applied 
to the thermopile over the two wires of the telephone circuit in multi- 
ple, the return being made through the ground. Properly arranged 
retardation coils prevent the short-circuiting of the voice currents, 
but allow the passage of the comparatively steady battery or dynamo 
currents. These thermopile systems have not been used in practice. 



CHAPTER XVII. 
SIGNALING IN COMMON BATTERY SYSTEMS. 

In the magneto system the subscriber signals the central office 
by turning his generator crank. In common battery systems he sig- 
nals by merely taking his receiver from its hook, or hanging it up 
again. Evidently, then, in common battery systems, there is some 
change in the electrical condition of the line brought about by the 
changing of the position of the hook. One of the requirements of 
all the systems in use is that current must flow over the metallic 
circuit of the line when the receiver is off its hook in order to ener- 
gize the transmitter. Since it is necessary to have a flow of cur- 
rent when the receiver is off its hook, and since, for the purpose 
of signaling, the condition must differ when the receiver is hung 
up, the plan has been adopted of having no current flowing on the 
line when the telephone is not in use. In order to accomplish this 
the hook switch when depressed maintains the circuit of the line 
open to direct currents and closed when raised. 

In common battery systems a different class of signals from those 
used in magneto systems is rendered available. In magneto sys- 
tems signals are almost universally of such type as employ a shut- 
ter which has to be restored either by hand or by some mechanical 
device. In other words, the signals are of such type as to prolong 
their display after the current which actuates them ceases. This is 
made necessary in order that the subscriber may not be forced to 
continually turn his hand generator until he secures the response 
of the operator. In common battery systems, however, as soon as 
a subscriber removes his receiver from its hook current begins to 
flow and continues to flow without further effort on his part until 
something happens at the central office. Therefore signals may be 
employed which will be displayed as long as the current which 
actuates them flows, and which will be obliterated as soon as the 
current flow ceases. 

Several types of mechanical signals operating on this principle 
have been produced and have been fairly successful. In .these the 
armature is simply made to lift a target within the range of vision 
of the operator and to hold it in its displayed position as long as 

277 



278 AMERICAN TELEPHONE PRACTICE. 

the current flows through its coil. As soon as the current ceases 
the target drops back, either by the force of gravity or of a light 
spring. 

A much better class of signal, however, for many reasons, is the 
miniature incandescent lamp, which is now quite universally used 
for signaling purposes in place of any of the forms of electric mechan- 
ical signal. The use of the incandescent lamp as a signal was prob- 
ably first proposed by Mr. J. J. O'Connell of Chicago. 

The advantages of the incandescent lamp over electro-mechan- 
ical signals are many, and among them may be mentioned the fol- 
lowing: They are much more compact than even the simplest elec- 
tro-mechanical signals ; they are free from mechanical complication ; 
they are automatic in operation, being always restored to their nor- 
mal condition by the cessation of current through them; they are 
capable of attracting the attention of the operator with more cer- 
tainty than the ordinary mechanical shutter; they are easily re- 
placed when destroyed; they are cheaper than almost any con- 
ceivable form of mechanical signal, and by the use of different col- 
ored glass lenses in front of them they may be used in the same 
board to indicate different kinds of information to the operator. In 
the early days of the use of lamps for telephone signaling purposes 
it was necessary to cite against the advantages pointed out the very- 
serious disadvantage brought about by the inability of lamp manu- 
facturers to produce- a uniform grade of miniature lamps. Many of 
the lamps furnished to operating companies proved utterly unfit 
for use., and, as a result, the difficulty in procuring good lamps in 
some cases caused the abandonment of the lamp signal system. In 
recent years, however, the lamp manufacturers have risen to the 
occasion and are producing lamps of such efficiency and uniform 
character as to leave little to be desired. 

To sum up the requirements thus far considered for enabling the 
subscriber to convey signals to the central office, it is seen that the 
signal at the central office is of the kind which continues displayed 
as long as the current through it continues, and that the control 
of these signals is affected by the position of the subscriber's hook 
switch, this switch allowing direct current to flow over the metallic 
circuit of the line when the hook is raised, but barring such current 
when depressed by the weight of the receiver. 

This condition brings about a difficulty when the problem of 
signaling the subscriber from the central office is considered. It 
is of course necessary to ring the subscriber's bell when his re- 



SIGNALING IN COMMON BATTERY SYSTEMS. 279 

ceiver is on its hook, and therefore at a time when there is no con- 
ductive path between the two sides of the line at his station. The 
fact that an alternating current may be made to pass through a 
circuit that is not conductively continuous affords the most ready 
solution to the problem, and is the one now almost universally 
adopted. An ordinary polarized ringer woun,d with a comparatively 
large number of turns is bridged across the two sides of the line 
in series with a condenser. The arrangement at the sub-station 
then becomes that shown in Fig. 236. The presence of the con- 



yjufrjg j5tafcc?i ~- 




FIG. 236.— SIMPLIFIED SUB-STATION APPARATUS. 

denser prevents the passage of direct currents across the line cir- 
cuit when the receiver is on its hook. An alternating current sent 
over the line from the central office will, however, find ready pas- 
sage through the condenser and will thus ring the bell. Thus 
without destroying the conditions necessary to enable the subscriber 
to signal the central office the sub-station bell is made responsive 
to ringing current sent by the operator to call the subscriber. 

Obviously, there are two methods of associating incandescent 
lamps with the circuits of subscribers' lines. The first of these, and 
without mature consideration the most desirable, is to place the 
lamp directly in the circuit of the subscriber's line and operate it 
automatically by the closure of the circuit of the line caused by the 



-o— 



+ 1 



li7te~ 

i 



iin. 



s Central Ojfice— 

FIG. 237.-LINE SIGNALING WITHOUT RELAY. 

raising of the subscriber's receiver from its hook. Such an arrange- 
ment is shown, stripped of all details, in Fig. 237, the subscriber's 
station being shown at the left connected by a metallic circuit line 



2>0 AMERICAN TELEPHONE PRACTICE. 

with the signaling apparatus at the central office. When the sub- 
scriber's receiver is on its hook the line circuit is open, and therefore 
no current will flow. As soon, however, as the receiver is removed, 
the circuit will be closed through his talking apparatus, thus allow- 
ing current to flow from the central office battery through the lamp, 
causing its illumination. 

The second. method is to have the lamp in a local circuit at the 
central office, this circuit being controlled by a relay. The relay 
coil with this arrangement is placed directly in the line circuit, and 
is therefore adapted to be operated by the current caused to flow 
when the subscriber removes his receiver from its hook. This ar- 
rangement is shown in Fig. 238 where the coil of the relay, R, is 

- 1 f 



Central Office^ 



I 

FIG. 238.— LINE SIGNALING WITH RELAY. 

placed in the circuit of the line instead of the lamp, as in Fig. 2yj. 
When the relay is energized by a current flowing over the circuit 
of the line it attracts its armature, thus causing the illumination of 
the lamp which is placed in the local circuit containing the battery 
and the relay contacts. 

Considering the first method — that is, where the lamp is placed 
directly in the circuit of the line, we are confronted by several rather 
serious objections. The resistance of no two subscribers' circuits 
are the same owing to the differences in the lengths of the lines and 
other causes, and therefore, unless some steps are taken to overcome 
this objection, no two lamps will receive the same current. The 
variation of the resistance of the lines will be such that the long 
lines would be of such high resistance as not to allow sufficient 
current to pass for the illumination of the lamp, while the shortest 
lines would be of such low resistance as to subject the lamp to an 
undue current. This featuure may be overcome by equalizing the 
resistance of the lines by placing resistance coils in series with the 
lamps on all except the longest lines, thus making the resistance 
of all lines practically equal to that of the longest. 



SIGNALING IN COMMON BATTERY SYSTEMS. 281 

Another difficulty brought about by the use of the lamp directly 
in the circuit of the line arises from the fact that lines are always 
more or less liable to crosses or short circuits, in which case the 
lamp would under ordinary circumstances be subjected to such a 
voltage as would cause it to burn out. It is this latter objection 
that has proved the most serious in practice, and has largely brought 
about the abandonment of this plan of associating the line lamp 
with the line. Of course, in underground systems, this objection 
has not been such a serious one. 

Although this plan has been universally abandoned, so far as the 
writer is aware, there seems to be a tendency recently developed 
to again revert to it. The expediency of the plan now seems to 
depend on what lamp manufacturers will be able to do in the way 
of manufacturing a lamp with sufficiently wide limits of operating 
conditions. It is evident that if a lamp could be built which would 
stand the full voltage of the battery without injury, the difficulty 
due to burnt-out lamps caused by short-circuiting the line -con- 
ductors would be obviated; in other words, a short circuit would 
do no harm to the lamp, but would simply cause its illumination 
to its full candle power. Under this condition the lamp would be 
self-protected. The other practical requirement for such a lamp 
would be that it should be sufficiently illuminated through a resist- 
ance equal to that of the longest line. 

Lamps have been built in small quantities adapted to fairly meet 
this requirement. A few such lamps tested by the writer had a re- 
sistance of 1 200 ohms, and were illuminated to full candle power 
by a pressure of 48 volts. In a system using such lamps this would 
therefore, be the pressure of the central office battery. These lamps 
proved capable of giving a signal good enough for practical pur- 
poses, even when placed in circuit with an 800-ohm line. As few 
lines in common battery exchanges ever have a greater resistance 
than this, it would seem that these lamps would be able to meet 
the requirements of practice in regard to their illumination over all 
ordinary lines, fairly well. Of course, the question as to the dura- 
bility of such lamps, and as to whether their manufacture in large 
quantities would be commercially possible or not, is yet to be de- 
termined. 

The method of operation of the line lamp by means of a relay, 
as shown in Fig. 238, is, however, eminently satisfactory, its only 
disadvantages being the first cost and subsequent maintenance of 
the line relay. It is doubtful, therefore, whether even the produc- 



282 



AMERICAN TELEPHONE PRACTICE. 



tion of a perfectly satisfactory lamp to be used directly in the line 
circuit would bring about the abandonment of the use of the line 
relay. 

In Fig. 239 is shown a more complete circuit of a common bat- 
tery line, as adapted to use in small exchanges, the jack, by which 
connection is made to the line for the purpose of conversation, hav- 
ing been added to the arrangement shown in Fig. 238. This jack, 
as will be seen, is of the cut-off type, being adapted, when a plug is 
inserted into it, to cut off the battery and the line relay, and thus 
extinguish the lamp. The operation, therefore, of this circuit from 
the time the subscriber called until the operator answered by insert- 
ing the plug is this: Upon removing his receiver from its hook the 
circuit of the line is completed, allowing current to flow from the 

I 






Zi?te~- 




Ceniral Office- 



FIG. 239.— LINE CIRCUIT. 



battery through the line relay, thus causing the operation of this 
relay which attracts its armature, and allows current to flow through 
the lamp. Upon inserting a plug into the jack the two sides of the 
signaling circuit are broken at the contact points, a and b, in the 
jack, thus breaking the circuit through the line relay, which, there- 
fore, allows its armature to fall back and extinguish the lamp. 

The methods by which the subscriber, by removing his receiver 
from its hook, is enabled to attract the attention of the central office 
operator, have now been discussed in general. It remains now to 
show how the subscriber by hanging up his receiver at the close of 
a conversation is again enabled to signal the operator. The old 
clearing-out drop of the magneto exchange has given place, in mod- 
ern common battery systems, to what are termed supervisory sig- 
nals. One of these signals is placed under the control of each sub- 



SIGNALING IN COMMON BATTERY SYSTEMS. 283 

scriber as soon as his line is connected to for the purpose of con- 
versation. To accomplish this result a signal (usually a lamp) is 
associated with each plug used by the operator, there being two 
such signals, therefore, for each pair of plugs used for connecting 
subscribers for conversation. The signal associated with the an- 
swering plug is therefore called the answering supervisory signal, 
and that with the calling plug the calling supervisory signal. The 
significance of the word "supervisory" will be obvious when it is 
stated that these signals enable the operator to supervise a connec- 
tion when made, since they keep her informed as to the condition of 
the connection, with regard to whether the called subscriber has 
responded; whether the subscribers are in communication; whether 
the conversation is finished, and whether one of the subscribers de- 
sires another connection. 

As soon as the connection is made with a line by means of a plug 
and cord the line signal is automatically taken from the control of 
the subscriber, and in its place is substituted a signal belonging to 
the particular plug and cord used in making the connection. Un- 
der these conditions, as long as the subscriber's receiver is off its 
hook — that is, as long as 'the subscriber is using his telephone, 
the supervisory signal will not be displayed. As soon, how- 
ever, as he hangs up his receiver, the signal will be displayed, 
thus informing the operator that the subscriber has finished 
the use of his telephone. The operation, therefore, of the two super- 
visory signals, associated with any cord circuit used in making a 
connection, is as follows: When an operator answers a call by in- 
serting the answering plug into the jack of the calling subscriber, 
the answering supervisory signal will not be displayed because that 
subscriber has his receiver removed from its hook. When the oper- 
ator inserts the calling plug into the line of the subscriber called for 
the calling supervisory signal will be displayed, because that sub- 
scriber has not yet removed his receiver from its hook. The calling 
supervisory signal will therefore serve as a ringing signal to the 
operator, she ringing the subscriber at short intervals until she 
knows that he has responded, by the retirement of the calling super- 
visory signal. Both supervisory signals will be undisplayed as long 
as the two subscribers are in conversation. When either subscriber 
finishes the conversation and hangs up his receiver the correspond- 
ing supervisory signal will be displayed, and the display of both 
such signals will convey to the operator the information that she 
is to take down the connection. Tf she sees one signal displayed 



284 AMERICAN TELEPHONE PRACTICE. 

while the other is undisplayed she will know that one of the sub- 
scribers probably desires another connection. It will be seen that 
this system of supervisory signaling is not dependent upon any vol- 
untary act upon the part of the subscriber, and for this reason the 
greatest unsatisfactory element of the old clearing-out signal, the 
inability to depend on the subscriber to turn his generator crank for 
the purpose of conveying the clearing-out signal, has been removed. 
Coming now to a general consideration of the means by which 
the supervisory signal is thus put under the control of the subscriber 
during a connection, reference is made to Fig. 240. At the left of 
this figure is shown the circuit of the subscriber's line, the line sig- 
nal having been omitted. At the right is shown a cord circuit 
equipped with a split repeating coil, between which is bridged the 
common battery for supplying current to the subscriber for talking, 
as in the Hayes system of common battery transmission. It is ob- 
vious that when two plugs are connected with the jacks of two lines 
the talking circuit of the two lines will be the same as that shown 



nr JL tine- ■=^W* r *t 5 « aj * m>^ = Zuie- Ik ~" r 




FIG. 240.— SUPERVISORY SIGNALING WITH PLUG-SEAT SWITCH. 

in Fig. 231, in Chapter XVI. 5 and S' are supervisory relays, one 
being connected in the sleeve strand of the answering plug, the other 
being similarly connected in the sleeve strand of the calling plug. 

When the armature of either of these relays, say S, is unattracted 
— that is, when no current is traversing the relay coil — the relay con- 
tacts will be closed, so that the corresponding lamp, L, would be 
lighted if its circuit were open at no other place. It is evident, how- 
ever, that when the plug is idle — that is, when it is not placed in the 
jack of the line — the relay, S, will be de-energized, which would 
cause the constant illumination of the lamp, L. This would be 
wasteful of current, wasteful of the life of the lamp, and confusing 
to the operator, because, under the condition of idleness, the 
lamp should not be displayed. For this reason, some means is 
necessary for preventing the illumination of the lamp when the 
plugs are idle. One of such means is shown in Fig. 240, where 
what is termed a plug-seat switch is employed. This is a switch 
associated with the seat in which the plug normally rests, so ar- 



SIGNALING IN COMMON BATTERY SYSTEMS. 285 

ranged that when the plug is in its seat the switch contacts are held 
open. These switch contacts are included in the circuit of the cor- 
responding supervisory lamp, and therefore when a plug is not in 
use the circuit of the supervisory lamp is held open at the plug- 
seat switch, although it is closed at the contacts of the relay. 

As soon, therefore, as the operator, in response to a call, raises 
the plug from its seat, the supervisory lamp, L, will be lighted on 
account of the closure of the plug-switch contacts, but it will im- 
mediately go out when the answering plug is thrust into the jack 
of the line, because that subscriber, having his receiver removed 
from its hook, current will flow though the supervisory relay, S, and 
open the local circuit of the lamp at the relay contacts. As long 
as the plug is in the jack the illumination of the lamp, L, depends 
only on the position of the armature of the relay, S, and it will be 
seen that this armature is under the control of the subscriber with 
which that plug is connected. As long, therefore, as the subscriber's 
receiver is removed from its hook the lamp, L, will not be illumi- 




L 
FIG. 241.-SUPERVISORY SIGNALING WITH THIRD STRAND IN CORD. 

nated, but as soon as he hangs up his receiver at the termination 
of a conversation, the relay armature will fall back and the lamp 
will be lighted to be put out again by the replacing of the answer- 
ing plug in the plug-seat switch. 

It will thus be seen that the circuit of each supervisory lamp is 
controlled by two separate pairs of contacts — one, the plug-seat 
switch, being under the control of the operator, and the other, the 
relay contacts, being under the control of the subscriber, after a 
connection has been made with a line. 

Another, and what is usually considered a better way of accom- 
plishing the same result, is shown in Fig. 241. Here the line circuit 
and apparatus is the same as that of Fig. 240, with the exception 
that an additional contact ring, a, has been added to the jack. The 
cord circuit is provided with three strands for each plug instead of 
two, a third contact, b, being provided on each plug for registering 
with the extra contact ring, a, of the jack. The connection from 



286 



AMERICAX TELEPHOXE PRACTICE. 



the tip and sleeve contacts of the plug to the repeating coil and bat- 
tery is the same as that in Fig. 240, the sleeve strand including the 
supervisory relay, 5. The third contact, 6, on the plug is con- 
nected through a third strand in the cord to one terminal of the 
supervisory lamp, L, the other terminal of which is connected to 
the back contact of the relay, 5, the armature of which is connected 
to the ungrounded terminal of the battery. 

Xo plug-seat switch is used in this case, but it is obvious that its 
equivalent exists in the contact, a, on the jack and b, on the plug, 
for as soon as the plug is inserted into the jack the registering of the 
contacts, a and b closes the normally open breach in the lamp cir- 
cuit, thus placing the supervisory lamp under the full control of the 
relay, S, which, as before, is governed by the subscriber. 

This scheme, with certain modifications, is almost universally used 
by the Bell companies and many of the Independent companies. 
The actual connection of the lamp itself is generally modified by 



T^n/Kpi 




FIG. 242.— SUPERVISORY SIGNALING WITH DIFFERENTIAL RELAY 



causing the relay to operate on the lamp by shunting it out of cir- 
cuit rather than by opening its circuit. These modifications will 
be fully pointed out in a subsequent chapter dealing with common 
battery circuits in practice. 

There is a radically different method of operating supervisory sig- 
nals which, however, causes the same code of signaling as already 
described to be put into effect. This system was designed by the 
writer, and has been put into extensive use in several large ex- 
changes in this country, including those of the Independent com- 
panies at Baltimore, Pittsburg, Scranton and other cities in the East. 
This system is interesting as being the only one the writer can call 
to mind where the supervisory lamp is controlled by a single pair 
of contacts, and also as perhaps embodying the simplest possible 
form of cord circuit for accomplish complete double-lamp super- 
vision. 

This system is shown in simplified form in Fig. 242. The ar- 



SIGNALING IN COMMON BATTERY SYSTEMS. 287 

rangement at the subscriber's station differs from those already 
pointed out in that the call-bell is connected between the lower con- 
tact of the hook and the ground in such manner as to ground the 
sleeve side of the line at the subscriber's station when the receiver 
is hung up. The removal of the subscriber's receiver from its hook 
operates the line signal in the usual manner, as pointed out in Fig. 
238. The plugs used in the cord circuit have two contacts only 
and the cords two conductors only. The two plugs belonging to 
a pair are inductively connected by means of a repeating coil, a sepa- 
rate battery being connected in the middle of each side of this coil, 
that pole of the two batteries which feeds the tip side of the cord 
circuit being grounded. In the tip strand of each cord is one wind- 
ing, r or r', of the supervisory relay, R or R' . Similarly connected 
in the sleeve strand of this cord is the other winding, s or s f , of the 
same relay. The two windings of each relay are concentric and 
wound in such manner, each having the same number of turns and 
resistances, as to produce no effect on the core when traversed by 
current flowing in series in the metallic -circuit of the line. The 
nature of the winding is also such as to produce practically no im- 
pedance to voice currents passing over the metallic circuit. Each 
supervisory relay controls by its armature the circuit of its super- 
visory lamp. It is obvious that under normal conditions when the 
plugs are not in use the armatures of the supervisory relays will be 
back, thus keeping the circuit of the lamp open. When a plug is 
inserted into a jack of the line, of which the receiver is removed 
from its hook, current will flow only through the metallic circuit 
of the line. Equal currents will then flow in opposite directions 
through the two coils of the supervisory relay, and no attraction 
of the armature will result. The lamp will therefore not be lighted. 
As soon, however, as either subscriber hangs up his receiver the 
metallic circuit of the line will be broken and a new circuit will be 
established, which may be traced from ground at the central office 
through the battery and the winding, s or s', of the supervisory re- 
lay, R or R', to the sleeve side of the line, thence to ground at the 
subscriber's station. Under this condition only one coil of the 
supervisory relay will be energized and the armature will be at- 
tracted, thus lighting the corresponding supervisory lamp. 

While this system is still largely used, it possesses an objection 
not found in those systems wherein the supervision is accomplished 
over the metallic circuit of the line without the use of ground. 
Ground connections at the subscribers' stations are somewhat ex- 



288 AMERICAN TELEPHONE PRACTICE. 

pensive to install and maintain, and in some cities trouble is experi- 
enced on account of earth currents caused principally by faulty 
ground returns in electric railway circuits. Sometimes differences 
of potential as high as 60 volts have been found in American cities 
between the ground at the central station and that at some of the 
outlying subscribers' stations. Of course, with such potentials as 
this, which are not constant, the use of grounded circuits for signal- 
ing becomes objectionable. Furthermore, this system does not lend 





FIG. 243.— SWITCH-BOARD LAMP. 

itself as readily to party line signaling as do some of the others 
which have been described. 

At first the lamps used for telephone purposes were adapted to 
screw sockets, the base being threaded in much the same manner 
as the present lamps used for commercial electric lighting. It was 
not long, however, before this form was changed to one in which 
the lamp was provided with two contact plates, arranged on oppo- 
site sides of the bulb, these plates forming the terminal of the fila- 
ment, and being adapted to slide into their sockets. In this case 
the sockets consisted of two springs arranged to properly register 



SIGNALING IN COMMON BATTERY SYSTEMS. 289 

with the contact terminals on the lamp. This is present practice. 
In order to economize in room with respect to the available space 
on the face of the board, the bulbs are made of small glass tubes 
about five-sixteenths of an inch in diameter. 

Types of switch-board lamps are shown in Figs. 243 and 244. In 
each of these the two contact terminals are seen on each side of the 
bulb. In the lamp shown in Fig. 243 these contact terminals pro- 
ject back of the bulb for a distance of about three-eighths of an inch 
and clamp between them a small wooden block which serves to 
make the structure more rigid. In the lamp shown in Fig. 244 
the wooden block is replaced by a kind of insulating cement resem- 
bling plaster of Paris, this cement being poured in after the ter- 
minals are fastened to the lamp. 

It will be seen that the lamp of Fig. 243 has a pointed tip or front, 
while that of Fig. 244 has a rounded front. The pointed front con- 
struction is faulty, because the point tends to obstruct the passage 
of light from the front where light is most needed. This point in 




FIG. 244.— SWITCH-BOARD LAMP. 

the front of the bulb is caused by the fact that the lamp is sealed 
at that end in manufacture. In the construction shown in Fie 
244 the lamp is sealed at the opposite end, the front of the bulb 
being previously rounded so as not to obstruct the passage of light 
from that end. 

Where lamps are used as line signals they are usually mounted 
in strips of 10 or 20 in such manner as to be immediately adjacent 
to the answering jack of the corresponding line. To facilitate this 
mounting the sockets into which the lamps are adapted to slide 
are arranged in strips of 10 or 20, the strips usually being composed 
of a single block of hard rubber, properly shaped and drilled for 
the reception of the lamps. 

The lamps are adapted to slide into place, so that their contact 
terminals will register with the contact springs in the sockets or 
lamp-jacks, as these sockets are usually called. In front of the lamp 
is placed a small lens, usually of opalescent glass, through which 
the light shines when the lamp is illuminated. Such a strip of jacks 
is shown in Fig. 245, which figure also shows a lamp and lamp cap. 

19 



200 



AMERICAN TELEPHONE PRACTICE. 



Telephone lamps are now most commonly built for 24-volts press- 
ure, although both higher and lower voltages are frequently used. 
The present tendency seems to be to increase the voltage rather than 
to diminish it, and many systems are being installed where the 48- 
volt lamps are used. At first lamps of 2 and 4 volts were employed, 
but for various reasons, not the least among which was the trouble 
in securing proper contacts at the various switch points for such 
low voltages, and the necessity for using low resistance conductors 
in order to effect the proper illumination of the lamps, the voltage 
Avas gradually increased as stated. 

Mr. A. V. Abbott, formerly of the Chicago Telephone Company, 
some years ago gave some interesting figures concerning the life 
of incandescent lamps in switch-board work, and mentions one case 
in which a lamp was flashed over a million times without showing 
serious sisfns of deterioration. His test seemed to indicate that for 



00 OO ,0 00 00 



FIG. 245.— STRIP OF LAMP JACKS. 



general service in switch-board work the average lamp will live for 
a period of about 1200 hours, although in laboratory tests a much 
longer life proved possible. He pointed out, as a result of his ob- 
servations, that, according to theory, the lamps used in subscribers' 
line circuits should last about 25 years, and those used in the cord 
circuits as supervisory or clearing-out lamps, from one to two years. 
He also says that such a life has already been obtained in the cord 
circuit lamps, but it is doubtful if the theoretical limit of the line 
lamp will ever be closely approximated. 

It is the experience of the writer that Mr. Abbott's figures for the 
average life of a lamp is very much too high, and that 500 hours 
of illumination is more nearly, correct than 1200. 

It has been said in this chapter that the lamp signal is almost 
universally used for line and supervisory signals in common battery 
exchanges. There are, however, certain exceptions to this general 
rule. In some very small common battery exchanges where but a 
small amount of current is available, and where for that reason it 



SIGNALING IN COMMON BATTERY SYSTEMS. 291 

becomes necessary to economize in current as much as possible, 
electro-mechanical signals are used, these taking their place in the 
circuit in the same relation as that usually occupied by the line or 
supervisory relays, where lamp signals are used. 

An electro-mechanical signal of this type is that used by the Kel- 
logg Company, shown in Fig. 246, and commonly termed the "grid- 
iron" signal. In this a small aluminum target, marked on its face 
with alternate black and white strips about one-eighth of an inch 
wide, is attached to the armature lever in such manner as to be 
moved in its vertical direction by the attraction of the armature 
through a distance just equal to the width of one of the black or 
white strips. Directly in front of the target is placed a black strip, 
having cut in it four horizontal slots equal in width to the width of 
the strips on the target, and equal in length to the width of the tar- 
get. When the magnet is de-energized the relative position of the 




FIG. 246.— GRIDIRON SIGNAL. 

target with respect to the front strip is such that the black strips on 
the target come directly in front of the openings in the front strip, 
thus presenting a continuous black surface to the eye of the oper- 
ator. When, however, the armature of the signal is attracted, the 
target is raised slightly so that the white strips come in front of the 
openings in the front strip, thus displaying four white horizontal 
patches where before all was black. The object of this "gridiron" 
arrangement is to secure the display of a white surface of consider- 
able size with a relatively small movement of the armature and 
target. 

Another form of signal during the past few years has been quite 
extensively applied by the Western Electric Company. This is 
shown in Fig. 247. In this the core of the magnet is clamped by 
means of a screw, a', to the mounting strip. Field between the core 
and the mounting strip is a U-shaped pole-piece, c\ in which to pivot 



292 



AMERICAN TELEPHONE PRACTICE. 



the rotating armature, e' . This armature normally hangs in the 
position shown in the upper cut of Fig. 247, but when attracted by 
the core of the magnet is drawn into the position shown in the 
middle cut of this figure. Carried on the same frame with the 
armature in such manner as to always partake of its movement is 
a target, /, made of aluminum, and stamped into the form of a seg- 







FIG. 247. 



ment of a sphere. Under normal circumstances this target lies in 
the position shown in the upper cut of Fig. 247, the weight of the 
armature, e\ serving to overbalance the weight of the target. When, 
however, the armature is attracted the target fills the opening, d, 
in the front plate, under which circumstances the target forms a con- 
spicuous signal. A front view of the three signals as mounted in 
practice is shown in the lower part of the figure, the signal in the 
middle being displayed while the other two are not. 



CHAPTER XVIII. 



COMMON BATTERY SWITCH-BOARDS FOR SMALL EXCHANGES. 



In the two preceding chapters are given the elements of circuit 
arrangement, and of apparatus by which voice transmission and the 
transmission of signals are effected in common battery exchanges. 
These elements were, in a large measure, separately considered, but 
from them the arrangement of circuits and apparatus for a complete 



' rv(2> 



SUB5 . 
STATION. 



SOB'S . 
STATION 



pCfo-v 

<§hr 1 I 



n> r 



n> r 




FIG. 248.— TYPICAL CIRCUITS FOR SMALL COMMON-BATTERY BOARD. 

switch-board, suitable for small exchanges, may easily be determined. 
In this chapter a few of the combinations of line and cord circuits, 
with their necessary details, will be considered; this subject-matter 
leading naturally to the discussion of the more complex multiple 
switch-board systems for common battery work. 

In Fig. 248 are shown line and cord circuits, suitable for a small 
switch-board working on the common battery plan with lamps for 
both line and supervisory signals. These circuits are typical of this 
class of switch-boards, as in parallel in this country to-day. In this 
figure two lines are shown, at the right and left-hand portion of the 

293 



294 AMERICAN TELEPHONE PRACTICE. 

page, with a cord circuit between them, and adapted to connecting 
these, or any other two lines, for conversation. The jack consists of 
a tip and a sleeve spring, which form the line terminals, and which 
normally rest against contacts leading to the ground on one side, and 
to the line relay on the other. A ring contact is also provided on the 
jack for the purpose of making connection with the third strand in 
the cord for supervisory signaling purposes. It is obvious that under 
normal circumstances the circuit of the line is held open at the sub- 
scriber's station, and that therefore the line relay does not operate 
because no current flows through it. When, however, the sub- 
scriber's circuit is closed by the raising of the receiver, current from 
the common battery flows through the line and causes the line relay 
to attract its armature. 

The two plugs of a pair have each three contacts, which may be 
designated in their order, tip, ring and sleeve. Of these the tip and 
ring are connected for voice transmission through the split repeating 
coil in accordance with the plan of the Hayes system, already referred 
to in connection with Fig. 231. Connected with the calling plug is 
the usual ringing and listening key, K', which needs no explanation. 
The operator's telephone circuit brought into play by the listening 
key, K, merits attention, however. Current is supplied to the opera- 
tor's transmitter and the primary of her induction coil from the 24- 
volt battery which supplies the cord and line circuits. In series w T ith 
the transmitter and primary winding is an impedance coil, C, having 
a resistance sufficient to cut down the current through the transmitter 
to a proper value for efficient working. With the solid back trans- 
mitter, used by the Bell companies, this resistance is about 140 ohms. 
A 2 M. F. condenser is bridged across the operator's primary cir- 
cuit in such manner as to shunt the transmitter and the primary coil, 
the purpose of this being to allow the free passage of fluctuating cur- 
rents through the primary coil when the transmitter is operated. 
Were the condenser not present the fluctuation set up by the trans- 
mitter would be forced to pass through the retardation coil, and their 
intensity would thus be greatly diminished. The operator's receiver 
and the secondary of her induction coil are associated with the 
primary circuit in the same manner as in magneto boards, with the 
exception that a condenser is placed in series with the receiver, the 
purpose of this being to prevent the operator from getting an undue 
click in the ear when she throws her listening key. 

Two supervisory relays, R and R', are connected in series in the 
sleeve strands of the answering and calling plugs, respectively. 



COMMON BATTERY SWITCH-BOARDS. 295 

When a plug is inserted in a jack it is evident that the corresponding 
supervisory relay is thus placed directly in the circuit over which 
battery current is fed to the line for conversation, and, therefore, the 
operation of this relay, when a connection is made with the line, will 
depend on whether or not the subscriber on that line has his receiver 
on or off its hook. These relays are usually shunted by a suitable 
non-inductive resistance, as shown, for the purpose of providing a 
non-inductive path around the relay coils for the voice currents. 
Sometimes condensers are substituted for these resistances. 

Between the third, or sleeve contact on the plug and the un- 
grounded side of the battery is connected the supervisory lamp, L, or 
L', this connection also including the normally closed back contacts 
of the corresponding supervisory relay. The sleeve contacts of the 
jacks are permanently grounded. The arrangement is therefore such 
that when a plug is inserted into a jack, and the supervisory relay is 
not operated, the lamp will be lighted, the current passing from 
ground at the cord circuit, through the battery, thence to the lamp, 
and through the third contact on the plug and the sleeve of the jack 
to ground. The operation of the supervisory relay caused by the sub- 
scriber removing his receiver from its hook opens the circuit of the 
lamp, thus keeping it extinguished as long as the subscriber's re- 
ceiver is in use. 

The operation of the system may now be understood. A subscriber, 
desiring a connection, removes his receiver from its hook, thus light- 
ing the line lamp. In response to this the operator inserts an answer- 
ing plug into the jack, thus cutting off both sides of the normal signal 
circuit at the jack, and establishing connection through the cord cir- 
cuit instead. Since the subscriber has his receiver off its hook, the 
relay, R, is actuated, and therefore the lamp, L, is not illuminated. By 
throwing her listening key, K, the operator bridges her telephone 
across the calling side of the cord circuit, and is enabled to con- 
verse with the calling subscriber in an obvious manner. Learning 
the number of the called subscriber, the operator will insert the call- 
ing plug into his jack and operate the ringing key, K', which will 
ring the bell of that subscriber. As soon as the calling plug was 
inserted in the called subscriber's line, the supervisory lamp, L'. 
became lighted, because, the subscriber's receiver being on its hook, 
no current passed through the supervisory relay, R', and therefore 
the circuit of the lamp was not opened at that point. As soon, how- 
ever, as the called subscriber responds, the supervisory relay. R', re- 
ceives current and the lamp is put out. 



296 



AMERICAN TELEPHONE PRACTICE. 



After conversation, when either subscriber hangs up his receiver 
the corresponding supervisory lamp will be lighted by the falling back 
of the corresponding supervisory relay armature, and when both of 
the lamps are lighted the operator will know that disconnection is 
desired. 

In Fig. 249 is shown the circuits of a common-battery system used 
for small exchanges based on the Stone system of transmission shown 
in Fig. 228. While this may not have gone into extended use, it is 
instructive, particularly with reference to the method of associating 
the electro-mechanical supervisory signals with the cord circuit. 



rSa 




FIG. 249.— SCRIBNER COMMON-BATTERY SYSTEM. 

The line signal is automatically operated by the removal of the 
subscriber's receiver from its hook, and is effaced by the insertion of 
a plug into the jack, which act opens the signal circuit at the jack. 
Current from the battery now circulates through the impedance coils, 
i and i\ and through the circuits of two connected lines, after the 
manner of the Stone system, already described. The listening and 
ringing key is so arranged that when the lever, d, is moved to the 
right the wedge, d\ will be forced between the springs, e and e', thus 
connecting the operator's telephone across the circuit. The springs 
and the wedge are so formed that the lever will remain in this posi- 
tion until moved by the operator. When pressed in the opposite di- 
rection, the wedge is forced between the springs, e 2 and e z , thus con- 



COMMON BATTERY SWITCH-BOARDS. 



297 



necting the generator with the calling plug. These springs are so 
formed that the wedge will be forced from between them when the 
pressure on the lever is released. 

Arranged in one side of the cord circuit in the ordinary manner are 
the supervisory signals, o and o\ these signals being constructed as 
shown in Fig. 250, which also gives a better view of the construction 
of the listening and ringing key. The indicators or shutters, a, are 
pivoted at their edges in cavities formed in the horizontal key-table. 
Each shutter is provided with a lug, a', upon which bears the free end 
of a flat spring, b, whose other end is fixed to the frame of a tubular 
magnet, arranged under the key-table. This spring tends to bring the 
indicator into a horizontal position, as shown in Fig. 250. The arma- 
ture, c\ of the tubular magnet carries an arm, c 3 , which, when the 




FIG. 250.— SUPERVISORY SIGNALS FOR SCRIBNER SYSTEM. 



armature is attracted, is thrown against the spring, b, thus pushing 
it out of engagement with the lug, a', on the shutter and allowing the 
shutter to fall from view. The lever, d, of the listening and ringing 
key is connected by a rod, /, with the springs, b, of the annunciators 
in such manner that when the lever is pressed into the listening posi- 
tion, as shown in Fig. 249, the springs, b, will be withdrawn from the 
shutters, thus producing the same effect as if the magnets were ener- 
gized, and allowing the shutters to drop out of sight. 

With this arrangement the keys are normally left in their listening 
positions, so that when an operator inserts an answering plug, k, 
into a jack in response to a call, she is at once placed in communica- 
tion with the subscriber. Having inserted the calling plug, k\ into 
the jack of the called subscriber, she moves the key to the ringing po- 
sition, and allows it to spring back to an intermediate position in 



298 



AMERICAN TELEPHONE PRACTICE. 



which neither the telephone nor the generator is connected with the 
cord circuit. This releases the springs, b, from the influence of the 
rod, f, but signal, o, will not be displayed, because current from the 
battery, t, is passing through the line of the calling subscriber, thus 
energizing its magnet and preventing its display. As the called sub- 
scriber has not yet responded, the signal, o', will be displayed because, 
sufficient current cannot pass through the high-resistance bell of the 
called subscriber to energize its magnet. As soon, however, as the 
called subscriber responds, current will pass through his line and the 
signal, o' , will be effaced. This condition will be maintained until 




FIG. 251.— CIRCUITS OF DEAN COMMON-BATTERY SWITCH-BOARD. 



one or both of the subscribers hang up their receivers, when the 
currents through the respective supervisory signal magnets will be 
cut off, their armatures will be released, and the shutters will be dis- 
played by being forced into a horizontal position. The operator will 
then withdraw the plugs, and will move the lever into the listening 
position in anticipation of the next call. This latter act will cause 
the rod, /, to pull the springs, b, out of engagement with the shutters, 
thus allowing them to fall. 

The Dean system of voice transmission, already described in con- 
nection with Fig. 234, is illustrated more in detail in Fig. 251, which 
represents the circuits as they have been used in a few small ex- 
changes. The impedance coil, /, at the central station has two wind- 



COMMON BATTERY SWITCH-BOARDS. *99 

ings, 7 and 8. One terminal of the coil, 7, is connected to the sleeve 
strand, while one terminal of the coil, 8, is similarly connected with 
the tip strand, the other terminals of these coils are connected to- 
gether at the point, /, which forms one terminal of the common bat- 
tery, B. In a similar manner the impedance coil, 7, at each sub- 
scriber's station is provided with two windings, connected re- 
spectively with the two sides of the line circuits, and having their 
other terminals joined at the point, /. The two windings on each 
coil consist of about 3000 turns of No. 22 silk-covered wire. As a 
result of this construction, the coils are of low ohmic resistance, 
especially when placed in parallel as they are with respect to the 
battery currents ; but they present a very high impedance to the voice 
currents flowing in the metallic circuit formed by the two line wires, 
for it is evident that in order to pass from one side of the circuit to 
the other these currents would necessarily pass through the two 
windings of the impedance coil in series. The currents from the 
battery, B, passing through the two windings in parallel, produce no 
magnetic effect upon the cores of the impedance coils, and therefore 
these coils are in a condition to offer a maximum amount of re- 
tardation. This is due to the fact that a mass of iron when in a 
neutral magnetic state is more susceptible to a magnetizing force 
than when the mass is polarized. 

At the sub-stations the supply circuit, after being united at the 
point, f, again divides and passes through the two halves of the 
primary circuit in multiple; but in this case two primary coils are 
provided, one in each side of the primary circuit, so that the changes 
in each side of the circuit may be utilized in producing an inductive 
effect upon the secondary coil. Thus, at station, A, the circuit di- 
vides at the point, f, one part passing through the side of the primary 
circuit containing the transmitter, and one of the primary coils, P' , 
and the other half passing through the branch containing the resist- 
ance, g, and the other primary coil, P. The two branches reunite 
at the point, f, which is grounded through the impedance coil, I 2 . 
The coil, g, has about the same resistance as the transmitter in its 
state of rest, so that the supply current will divide equally between 
the two halves of the primary circuit, and therefore normally pro- 
duce no magnetization of the core. A decrease in the resistance of 
the transmitter will cause a greater current to flow through the 
primary coil, P\ and a correspondingly less current through the 
primary coil, P. 

As the two primary coils in this circuit are oppositely wound, 



300 AMERICAN TELEPHONE PRACTICE. 

a decrease of currrent in one of them will produce the same induc- 
tive effect on the secondary as an increase in the other, and when 
these two effects take place simultaneously in the primary coils, the 
inductive effects upon the secondary coil are added. An increase 
in the transmitter resistance will in the same manner induce a cur- 
rent in the opposite direction in the secondary. 

Both limbs of each line circuit terminate in contacts on the hook- 
switch, so that when the hook is raised the connection is completed 
from the line wires through the telephone apparatus already de- 
scribed. When the hook is down, one limb of the line is left open 
and the other is closed to ground through a high-resistance polarized 
bell. 

At the central office the circuits are as already described, with 
the addition of the line annunciators, K and K\ and the clearing-out 
or supervisory signals, K 2 and K z . The operator's talking set is 
adapted to be bridged across the cord circuit by the listening key, 
while the generator may be connected between the ground and the 
tip of the calling plug. 

Assuming the apparatus to be in its normal position, when the 
subscriber at the left of the figure desires a connection with the 
subscriber at the right, he raises his receiver. This act grounds both 
sides of his line through his station apparatus. A current from the 
battery, B, thus flows through the drop, K, to the two sides of the 
line in multiple, by virtue of the fact that the tip- and sleeve-springs 
of the jack rest upon a common anvil. The current flows through 
the two sides of the subscriber's circuit in multiple, and to ground, 
and is of sufficient strength to cause the annunciator, K, to raise its 
target. The operator seeing the signal inserts the answering plug, 
thus cutting off the circuit through the annunciator, K, and allow- 
ing its target to assume its normal position. 

The circuits are now completed from the battery, B, through the 
two halves of the impedance coil, and to ground at the subscriber's 
station, as already described. The operator then bridges her tele- 
phone set across the cord circuit, and communicates with the sub- 
scriber. She inserts the calling plug into the jack of the line called 
for, and depresses the ringing key, which connects one terminal of 
the grounded generator with the tip strand of the cord, and therefore 
with one side of the line. A current flows from the generator to 
ground at the subscriber's station, and operates the polarized bell. 
That subscriber then removes* his receiver from the hook, and the 



COMMON BATTERY SWITCH-BOARDS. 



301 



two converse over the metallic circuit formed by the two connected 
lines. 

While the subscribers' receivers are removed from the hooks 
the current from battery, B, flowing through the sleeve strand of the 
cord circuit energizes the magnets of the clearing-out anunciators, 
K 2 and K 3 , and causes them to lift their targets. As soon as either 
subscriber hangs up his receiver this current ceases to flow, because 
the line wire, with which the sleeve strand is connected, is opened at 
the hook. This allows the target of the annunciator, K 2 or K 3i to 
fall, showing that that subscriber has ceased to use his insrument. 

This represents, perhaps, the highest development attained in any 
of the methods for centralizing all sources of energy in telephone 
systems by feeding over the two sides of the line in multiple. 

A line circuit wherein the lamp signal is connected directly in the 
line instead of in the local circuit of a relay, as is usual, is shown in 
Fig. 2^2. This is also interesting as showing a practical application 



UlHC LXMf» 




A^SJACK 



STATION^ 

FIG. 252.— LINE CIRCUIT WITH ELECTROLYTIC CELL. 

of the scheme already mentioned of placing a secondary battery at 

the sub-station charged from the central office over the line circuit. 

In this the subscriber's apparatus is shown at the left, and the 
central-office apparatus at the right. The line wire, 2, forming one 
side of a metallic circuit, is connected with the tip-spring of the jack, 
and passes through an incandescent lamp, /, and through an inductive 
resistance, h, to one pole of a battery, B. The other side, 3, of the 
metallic circuit passes through an inductive resistance, g, to the other 
pole of the same battery. When the subscriber's receiver is on its 
hook the circuit at the subscriber's station between the two sides of 
the line wire is completed only through the high-resistance call-bell, 
e, and as this bell has a resistance of about 1000 ohms, the current 



302 AMERICAN TELEPHONE PRACTICE. 

from the battery through the line circuit is not sufficient to illuminate 
the lamp, /. When, however, the subscriber's receiver is removed 
from its hook a circuit of low resistance is closed in parallel with the 
bell magnets, e, this circuit including the secondary windings of the 
induction coil, and the receiver in series. As this circuit may readily 
be made less than 40 ohms, sufficient current will be allowed to flow 
from the battery to illuminate the signal, and thus attract the opera- 
tor's attention. 

Whatever current passes through the bell magnets from the bat- 
tery at the central office, must also pass through the battery, d, at the 
sub-station. This consists of two cells of storage battery of the 
Plante type. Whenever the apparatus at the subscriber's station is 
not in use this battery will therefore be receiving a charge from fhe 
central-office source, the strength of the latter and the resistance of 
the circuits being so proportioned that the storage cell will receive 
a constant charging current of about .02 of an ampere. When the 
subscriber's apparatus is put in use, however, the battery is thrown 
in a local circuit including the primary winding, p, and the trans- 
mitter, and will then perform the functions of an ordinary primary 
battery in connection with the transmitter. The alternative func- 
tions which this battery, d, may perform are interesting. It is well 
known that if a storage cell of the Plante type becomes almost or 
quite discharged it will develop a counter E. M. F., when a current 
is sent through it in the direction necessary to charge it, and that 
this counter E. M. F. will be very nearly equal to the E. M. F. of a 
similar cell fully charged. Supposing, now, that from some cause 
or other the cell, d, becomes discharged to such an extent that it is 
incapable of furnishing enough current to operate with the trans- 
mitter in the usual manner. In this case, when the receiver is 
raised, the current from the battery at central, which tends to pass 
through the storage battery, will meet with a considerable counter 
E. M. F., which will compel most of the current to pass from the 
line wire, 2, through the secondary, S, receiver, hook lever, trans- 
mitter and primary, P, to the line w r ire, 3. 

The transmitter will therefore receive current from the battery, i, 
sufficient to operate it, and yet it will be operating with all the ad- 
vantages to be derived from a local circuit and induction coil; for, 
although the current operating it comes from the central office, any 
fluctuations in this current caused by the transmitter will pass 
through the low-resistance battery, d, which will act in this case 
very much in the same manner as a condenser. 



COMMON BATTERY SWITCH-BOARDS. 



303 



The same general designs of switch-board cabinets as are used 
in small magneto-switch-boards are often made to serve for com- 
mon batten- boards, their dimensions and construction being changed 




FIG. 253— COMMON-BATTERY SWITCH-BOARD FOR SMALL EXCHANGES. 



sufficiently to accommodate the different class of apparatus. A com- 
plete switch-board adaptable to exchanges, having not over one 
hundred lines, is shown in Fig. 253. 



CHAPTER XIX. 

COMMON BATTERY SUB-STATION EQUIPMENT. 

The essential features of the sub-station equipment for common 
battery work are the speech-receiving and transmitting apparatus, 
or receiver and transmitter ; the call receiving apparatus, or ringer, 
the switch-hook for alternately bringing the talking apparatus and 
the call receiving apparatus in proper relation with the line ; and a 
device, usually a condenser, for preventing direct currents from flow- 
ing over the metallic circuit of the line when the telephone is not in 
use, but adapted to allow alternating currents to pass for the purpose 
of ringing the bell. 

It has been shown in the three preceding chapters that the condi- 
tions required of the sub-station in order to bring about the oper- 




FIG. 254.— SIMPLE SUB-STATION CIRCUIT. 

ation of the various signals at the central office, are that the circuit 
of the line, when the telephone is not in use, shall be open to direct 
currents, at the same time allowing alternating currents to pass for 
the purpose of ringing the subscriber's bell. When the telephone is 
not in use the circuit between the two sides of the line must be com- 
plete with respect to both direct and alternating currents. 

The simplest form of circuit for the subscriber's station adapted to 
meet these requirements, is shown diagrammatically in Fig. 254. In 
this the talking apparatus consists of the transmitter and receiver, 
placed directly across the two limbs of the line when the hook is 
raised. The call bell is permanently bridged across the line, no con- 
denser being used in its circuit, and, in order to prevent the opera- 
tion of the signals at the central office by the flow of direct current 
through the call bell, the magnets of the latter are wound to a very 
high resistance, say 10,000 ohms. With this arrangement the relays 



COMMON BATTERY SUB-STATION EQUIPMENT. 305 

at the central office would necessarily be adjusted so as not to work 
through 10,000 ohms, but to respond properly when the shunt cir- 
cuit through the transmitter and receiver was closed around the call 
bell, as when the telephone is in use. 

This was the arrangement first proposed, and used to some extent 
in early common battery work. It proved faulty, however, for the 
following reasons: The constant flow of current from the central 
office battery through the call bells made necessary a marginal ad- 
justment of the relays at the central office, and also proved a con- 
stant drain on the storage batteries, especially severe when a large 
number of lines were served. 

These defects are removed by placing a condenser in series with 
the bell, in which case the extremely high winding of the coils is un- 
necessary, 1000 ohms being ample. The circuit arrangement with the 
condenser added then becomes that shown in Fig. 255. 

While the addition of the condenser in this manner removes com- 




FIG. 255.-SIMPLE SUB-STATION CIRCUIT WITH CONDENSER. 

pletely the difficulties due to the waste of current and to the marginal 
adjustment of the central office relays, it leaves several undesirable 
features with respect to the talking apparatus. This is due to the 
fact that the direct current which supplies the transmitter passes also 
through the receiver coils. Unless, therefore, a receiver is properly 
"poled;" that is, placed in the circuit of the line in such manner 
that the steady flow of current through it increases rather than 
diminishes the strength of its magnets, a serious loss of talking 
efficiency results. Very often when the receiver is wrongly placed 
in the line, the flow of current through it proves about sufficient to 
neutralize the effect of the permanent magnets, thus almost com- 
pletely destroying the effectiveness of the receiver. This disad- 
vantage does not exist if the receivers are placed in the line so that 
the current strengthens rather than weakens their pull on their dia- 
phragms. This can be effected by marking the positive and negative 
terminals of the receiver so that by due care the installers and in- 
spectors may properly connect them. Tt frequently happens, how- 
20 



306 



AMERICAN TELEPHOXE PRACTICE. 



ever, that in making changes in line connections, perhaps in a man- 
hole or on a pole, the two sides of the line will be transposed, which, 
of course, subjects a receiver, previously connected properly, to cur- 
rent in the wrong direction and makes necessary a trip of the in- 
spector to that station. 

This arrangement has another objection in that a receiver properly 
adjusted for maximum efficiency on a line having, a certain current 
strength, may not be properly adjusted for a line having a stronger 
current. Frequently when placed on a very short and therefore low 
resistance line, the magnetic attraction of the cores is sufficient to 
pull the diaphragm into contact with the pole pieces, thus destroying 
all possibility of its vibrating. 

The prevention of the flow of direct current through the receiver 
has been solved in a number of ways, perhaps the most common way 
being that employed in instruments made by the Western Electric 




FIG. 256.— WESTERN ELECTRIC SUB-STATION CIRCUIT. 



Company for the various Bell companies. In this arrangement, 
which is shown in Fig. 256, the receiver is included in a local cir- 
cuit which also includes, when the hook is raised, one winding of an 
induction coil, and a two-microfarad condenser, which is also em- 
ployed for transmitting the ringing currents. When the hook is de- 
pressed by the weight of the receiver there is no path for direct cur- 
rents between the limbs, a and b, of the line on account of the pres- 
ence of the condenser. The only path for alternating currents is 
that through the condenser and bell. When the receiver is removed 
from the hook, however, direct current may flow from the central 
office over the limb, a, of the line through the 17-ohm winding of the 
induction coil and the transmitter to the limb, b, of the line. This 
flow of current causes the operation of relays for signaling at the 
central office, and also supplies the transmitter with direct current 
for talking. When the transmitter is actuated by sound waves it will 
cause undulations in the current flowing in the line which, passing 



COMMON BATTERY SUB-STATION EQUIPMENT. 



307 



through the translating devices at the central office will cause cor- 
responding undulations in the receiving line, and thus effect the 
transmission of speech. 

When a station is receiving speech the fluctuating currents caused 
by the operation of the transmitter at a different station, will pass 
from the limb, a, to the line through the 17-ohm induction coil, and 
the transmitter to the limb, b, of the line, and by induction between 
the two windings of the induction coil, these fluctuations will be re- 
peated into the local circuit containing the receiver, the 30-ohm wind- 
ing of the induction coil, the condenser and the transmitter. 

The manner in which this circuit is supposed to operate is ex- 
plained by Mr. W. W. Dean in the following paragraphs, in con- 
nection with a diagram of which Fig. 257 is a reproduction : 

"The secondary coil of the induction coil, S, measures 17 ohms, 




FIG. 257.— WESTERN ELECTRIC SUB-STATION CIRCUIT. 



and has 1700 convolutions of wire; the primary circuit, P, measures 
30 ohms, and has 1400 convolutions of wire. The ends of the pri- 
mary and secondary coils, marked 0, are the outside ends, and the 
ones marked i are inside ends ; both coils are wound in the same di- 
rection on the core. The action of this current is as follows: Sup- 
pose the transmitter, T, is at rest and has a certain fixed resistance. 
Current will flow from the positive side of the battery over the line 
through the secondary winding of the induction coil in the direction 
of the arrow, No. 1, through the hook contact, transmitter, T, to the 
other side of the line, to the negative side of the battery. Between 
the points, x and y, there will be a certain electrical potential, the 
condenser, C, will be charged to the same potential that exists be- 
tween the points, x and y, and the direction of this charge will be as 



308 AMERICAN TELEPHONE PRACTICE. 

indicated in the sketch. In speaking into the transmitter, 7\ a sud- 
den lowering of its resistance is caused. The potential between the 
points, x and y, w T ill be reduced ; in other words it will be lower 
than the charge existing in the condenser, C. A discharge will now 
take place from the condenser through the primary, P, of the induc- 
tion coil, through the receiver, R, the hook, the transmitter, T, to the 
negative side of the condenser. This discharge of current will be in 
the direction indicated by arrow Xo. 2. The flow of current through 
the primary, P, as indicated, will cause a current to be induced in 
the secondary winding, S, in the direction of the arrow No. 3. This 
current, on account of the "step-up" effect of the induction coil, will 
be of a higher potential than the original current, and, as it is in 
the same direction as the current indicated by arrow No. 1, will 
augment it. When the transmitter, T, returns to its normal position, 
the potential will be again raised between the points, x and y. Cur- 




^yT'"' ' "£>■ 



FIG. 258.— STROMBERG-CARLSON SUB-STATION CIRCUIT. 

rent will, therefore, flow into the condenser from the point, y, 
through the hook to receiver, R } through the primary winding, P, 
in a direction opposed to arrow No. 2, to condenser, C, to the point, x. 
An induced current will be generated in the secondary, S, in a di- 
rection opposed to arrow No. 3. This current will oppose the main 
line current flowing in the direction of arrow No. 1, but on account 
of the increase in resistance of the transmitter, T, this current is on 
the decrease, and the last mentioned induced current will still fur- 
ther tend to decrease it. It will thus be seen that the current changes 
taking place in the telephone line are of much greater range than 
they would be if induction coil were not used. 

"In order to prove that the action of the coil augments the trans- 
mission, connect a switch so that the terminals of the secondary coil 
can be instantaneously reversed, or ? in other words, change the 
relative positions of the points, and i, of the coil, S, and a differ- 
ence in volume of transmission of about 50 per cent, will be noticed." 

An additional function of the induction coil and the local circuit 



COMMON BATTERY SUB-STATION EQUIPMENT. 



309 



in the Western Electric Company's arrangement is that of putting 
the receiver in inductive relation to the line without subjecting it to 
the passage of direct current through it. 

In Fig. 258 is shown the current arrangement of the common bat- 
tery sets manufactured by the Stromberg-Carlson Telephone Manu- 
facturing Company. In this the induction coil has one of its wind- 



m 



1 



FIG. 259.— SIMPLIFIED KELLOGG SUB-STATION CIRCUIT. 

ings placed directly in series with the transmitter when the hook is 
up, this path shunting the path containing the bell and condenser. 
The receiver is placed in an entirely local circuit in series with the 
other winding of the induction coil. This arrangement accomplishes 
the removal of the receiver from the action of direct current in a 
very simple manner. 

The method by which the Kellogg Company places the receiver 
in proper relation with the line without subjecting it to the passage 
of direct current is shown in simplified form in Fig. 259, which shows 
the condition when the hook is raised. In this the transmitter is 




FIG. 260.— KELLOGG SUB-STATION CIRCUIT. 

placed in series across the line with an impedance coil of low resist- 
ance, but of high retardation. Direct current from the central office, 
therefore, flows readily through this path for the operation of the 
signals and for the supply of current to the transmitter. Around the 
impedance coil is placed a shunting circuit containing the receiver 
and a two-microfarad condenser, and through this path, instead 



310 



AMERICAN TELEPHONE PRACTICE. 



of through the impedance coil, the fluctuating voice currents pass. 
It might be thought that the presence of the impedance coil would 
materially reduce the receiving efficiency of the station, but what- 
ever reduction of efficiency does occur due to its presence is so slight 
as to be unnoticeable even by an expert. 

In Fig. 260 is shown the actual working connections of this ar- 
rangement through the hook switch by means of which the bell or 




FIG. 261.— COMMON BATTERY WALL SET— CLOSED. 



the talking apparatus are alternately brought into service. It will 
be seen that when the hook is raised the conditions shown in Fig. 
259 exist. When, however, the hook is depressed the transmitter 
circuit is open, as is also the circuit of the impedance coil, while the 
receiver is short-circuited; the only circuit across the line is that 
through the bell and the condenser in series, which is the condition 
required by modern central office circuits. 

The various Bell companies employ in their ringing circuit a 



COMMON BATTERY SUB-STATION EQUIPMENT. 



311 



bell wound to iooo ohms and a two-mocrofarad condenser. This 
arrangement gives thoroughly satisfactory results and has been 
largely adopted among the various Independent companies. It is 
found, however, that almost equally satisfactory results may be 
obtained by the use of a 500-ohm bell with a two-microfarad con- 
denser. When, however, the capacity of the condenser and the 
resistance of the bell are both materially reduced from that of the 




FIG. 262.-COMMON BATTERY WALL SET-OPEN. 



Bell standard a decided falling off in efficiency is found; for instance, 
a 500-ohm bell with a one-microfarad condenser, while operative, 
does not give satisfactory ringing with the average central office 
generator. Many of the Independent companies are therefore 
using a two-microfarad condenser with a 500-ohm bell, the bell 
being of the usual long-core type and wound for a maximum num- 
ber of turns by using as large a wire as the winding space will 
permit. 



312 



AMERICAN TELEPHONE PRACTICE. 



As illustrative of the arrangement of parts in a modern common 
battery wall telephone, that of the Kellogg Company may be taken 
from among numerous good designs. This is shown in Figs. 261 
and 262, the latter figure showing the box opened for inspection or 
repairs. The ringer is mounted on the movable portion of the 
box, connection to it being made through the hinges. The hook 
retardation coil and various binding posts are mounted on the front 
face of the condenser receptacle, which is permanently secured to 




FIG. 263.— DETAIL OF HOOK AND CORD MOUNTING. 



the back board. In order to allow the box when opened to swing 
clear of the hook lever, the hook escutcheon is split, part of it being 
secured to the condenser receptacle and part of it to the movable 
box. The receiver cord is carried through a hole in the stationary 
part of the escutcheon, in order to prevent its being pinched by a 
careless closing of the box. These features are well shown in Fig. 
263. 



CHAPTER XX. 

THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 

In the development of the common battery multiple switch-board 
the tendency has been, on account of the great cost of the multiple 
jacks and cables, and further, on account of the ever-increasing ne- 
cessity to economize room in the jack space, to simplify the multiple 
jack, reducing the number of its contacts and the number of wires 
for each line in the multiple cable. We have seen that in the old 
branch-terminal magneto-multiple board each jack had five con- 
tacts, three springs and two sleeves or rings. Two of these con- 
tacts were strapped together in each jack, thereby making them, 
electrically speaking, one contact, while another of the contacts in 
each jack was connected to a common or ground wire. The circuit 
was such as to necessitate the use of three individual wires besides 
the common wire, all running through the entire length of the mul- 
tiple board and connected to contacts in a jack on each section. 

In some of the early common battery systems the circuits used 
were of such complexity as to require as many as four individual 
wires in the multiple cables for each line, together with one or more 
common wires. 

In the common battery multiple switch-board, universally adopted 
by the Bell companies and manufactured by the Western Electric 
Company, the circuit has been so simplified as to require only three 
contacts in the jack. Three wires, individual to each line, run 
through the multiple board connected respectively to the three con- 
tacts of each spring-jack belonging to that line. 

Recent developments have produced common battery switch- 
boards wherein only two contacts are required in the multiple jacks, 
and consequently but two wires for each line connecting with these 
jacks in the multiple board. On account of the keen rivalry be- 
tween manufacturers employing three wires for each line in the 
multiple cable and those employing but two, the names, "two-wire 
multiple switch-board" and "three-wire multiple switch-board," have 
recently come into use. These names signify, of course, the num- 
ber of wires, individual to each line, necessary to be carried through- 

313 



314 



AMERICAX TELEPHOXE PRACTICE. 



out the sections in order to make connections with the contacts of 
each multiple jack. 

Typical of all three-wire switch-boards is that of the Western 
Electric Company, used by the Bell Telephone companies and 
adopted as the standard by those companies. The line circuit in 
this system is shown in Fig. 264, which also includes the apparatus 
at the subscribers' stations shown at the right of the figure. The 
apparatus at the subscriber's station is, as will be seen, that already 
described in a previous chapter as being the standard Bell sub-sta- 
tion equipment. It may be well, however, to reiterate that when the 
receiver is on its hook the circuit of the line is open on account of 
the condenser, while, when the receiver is removed from the hook, 
the circuit of the line is closed through the talking apparatus. 

At the central office the two sides of the line pass to the tip and 
ring contact springs, respectively, of each multiple jack and of the 




jo 

—— ■ W STATION 




FIG. 264.— WESTERX ELECTRIC MULTIPLE SWITCH-BOARD LINE CIRCUIT 



answering jack, after which they connect respectively with two 
back contacts of a relay, c, called the cut-off relay. These contacts 
normally rest against two movable levers, of which the one con- 
nected with the tip side of the line is connected to the grounded 
side of the central office battery, and the one connected with the 
ring side of the line to the ungrounded side of the battery through 
the coil of the line relay, L. The winding of the cut-off relay is in- 
cluded in a circuit between the grounded side of the battery and a 
wire extending to all of the sleeve rings of the jacks. 

The line relay, therefore, normally stands ready to be operated 
by current flowing in the metallic circuit of the line, and when so 
operated lights the line lamp, S, by the attraction of its armature. 
This lamp draws current from the same battery that supplies current 
to the relays. It is evident, therefore, that the line relay is under 
the control of the subscriber, who, by removing his receiver from 
its hook, will close the circuit of the line, allowing current to flow 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 315 

from the battery through the line relay. At all times, however, 
when the subscriber's receiver is on its hook the circuit of the line 
is held open to direct currents by the condenser at the subscriber's 
station, thus preventing the operation of the line relay. 

The cut-off relay, being in an entirely local circuit, is under the 
control of the operator in a manner which will be pointed out when 
the cord circuit is considered. This relay, as will be seen by refer- 
ence to Fig. 264, serves, when operated, to sever the signaling appa- 
ratus and battery from both sides of the line circuit, thus rendering 
the line relay and lamp irresponsive to any movement of the sub- 
scriber's hook. 

One of the cord circuits is shown in Fig. 265. In this the answer- 
ing and calling plugs are shown at the extreme left and right-hand 
portions, respectively. Each plug has three contacts, two of which 




FIG. 265.— WESTERN ELECTRIC MULTIPLE SWITCH-BOARD CORD CIRCUIT. 



— the tip and ring — are connected, as indicated by heavy lines, 
so as to complete the talking circuit between the two plugs. The 
third contact of each plug is connected through the supervisory 
signaling apparatus to the live side of the battery, as shown. The 
tip and ring contacts of the answering plug are connected through 
the two halves of one side of the repeating coil, between which is 
connected the battery. In the same manner the tip and ring strands 
of the calling plug are connected by the two halves of the other 
winding of the repeating coil, between which is connected the same 
battery. This battery is the same as that used to supply current 
to the line signals, as indicated in Fig. 264. 

The repeating coil (there is one for each pair of cords and plugs) 
has four windings, each having approximately 3300 turns, all of the 
windings being on the same magnetic core. It is evident that these 
connections allow direct current from the storage battery to pass 



316 AMERICAN TELEPHONE PRACTICE. 

out over the metallic circuits of two connected lines to energize the 
sub-station transmitters, in accordance with the system of Hayes, 
already described in theory. In this way conversation between 
two connected subscribers is made possible. 

In the ring strand of the cord attached to each plug is connected 
a supervisory relay, R or R', which is energized, as is readily seen, 
only while current is flowing over a line with which the correspond- 
ing plug is connected, which occurs only while the receiver of that 
line is off its hook. Each relay controls the illumination of the su- 
pervisory lamp corresponding to the plug to which the relay be- 
longs. When a plug is inserted in a jack the relay, when actuated, 
serves to close a 40-ohm shunt about the lamp which prevents its 
illumination. Current is fed from the non-grounded side of the 
battery through an 83-ohm resistance coil to each lamp, the return 
side of this circuit being made, when the plug is inserted in a jack, 
through the cut-off relay to earth. Under normal circumstances, 
therefore, when the plug is not inserted in a jack the supervisory 
relay will remain unlighted even though unshunted, because the 
circuit to the grounded side of the battery is broken. When a plug 
is inserted into a jack, however, the lamp will be lighted as long 
as the supervisory relay is not actuated. Since the actuation of the 
supervisory relay depends upon the subscriber with whose line the 
plug is connected, having his telephone off the hook during con- 
versation, it follows that when a connection is made with a line the 
supervisory lamp will be shunted while the subscriber's receiver is 
off its hook, and therefore the lamp will not be illuminated. If, 
however, the subscriber's receiver is on its hook the supervisory 
relay will not be energized and the shunt will not exist about the 
lamp, which will therefore be lighted. 

A ringing and listening key are shown at the right-hand portion 
of this figure, and need no description. The operator's transmitter 
receives current constantly from the battery through a 140-ohm re- 
tardation coil and the primary of the induction coil. Bridged across 
the circuit from ground to a point between the transmitter and the 
retardation coil is a two-microfarad condenser for the purpose of 
allowing the fluctuation set up by the transmitter to circulate in 
the circuit formed by the condenser and the primary winding of the 
induction coil without passing through the retardation coil. The 
connections of the secondary winding of the operator's induction 
coil and the operator's head receiver are made in the usual man- 
ner. Bridged across the secondary side of the operator's talking 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 317 

circuit is an order wire key adapted to make connection between 
the operator's telephone set and an order-wire circuit, for the pur- 




pose of allowing the operator to communicate with some other oper- 
ator, usually at a distant exchange, for purposes that will be pointed 



318 AMERICAN TELEPHONE PRACTICE. 

out in subsequent chapters. The supervisory relays are shunted in 
all cases where they are placed in the talking circuit by a non-in- 
ductive resistance of about 30 ohms, this resistance being wound 
on the relay spool in addition to the active winding. A non-in- 
ductive path for the voice currents is thus afforded around the 
relay windows. 

In Fig. 266 the cord circuit of Fig. 265 is shown connecting two of 
the line circuits of Fig. 264. While the arrangement of the appa- 
ratus of the line circuits of this latter figure differ slightly in appear- 
ance, it will be seen that the actual circuit connections are the same, 
the change having been made in the diagrammatic arrangement for 
the purpose of giving a clearer understanding. 

Considering the line at the left to be thai of the calling subscriber, 
and the line at the right that of the called subscriber, the operation 
is easily understood in view of what has already been said. With 
both plugs removed from the jacks, the raising of the calling sub- 
scriber's receiver from its hook illuminates the lamp of that line, 
which remains lighted until the operator answers by the insertion 
of the answering plug into the answering jack. The insertion of 
this plug not only establishes the circuit over which speech is trans- 
mitted, but the third contact on the plug engages the sleeve of the 
jack, thus allowing current to flow from the live side of battery 
through the third strand of the cord and the third wire of the jack 
to the coil of the cut-off relay and to ground. This operates the 
cut-off relay which breaks the circuit between both sides of the line 
and the line-signaling apparatus. 

By means of her listening key the operator is then enabled to 
converse with the subscriber through the windings of the split- 
repeating coil in the cord circuit. 

The operator before inserting the calling plug into the multiple 
jack of the called subscriber must ascertain whether or not the line 
of that subscriber is busy, and upon touching the tip of the calling 
plug to the sleeve contact of the multiple jack of the called line she 
will get a click in her head telephone if the line is busy and silence 
if it is free. The reason for this is as follows: If the line is not 
connected to, at another section, all of the test contacts of the jacks 
of that line will be at the same potential as that of the earth, and 
when the operator applies the tip of the plug which also is con- 
nected to ground to such a contact no current will flow, because there 
is no source of electro-motive force in the circuit. If, on the other 
hand, the line is connected to, at another section, as would be the 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 319 

case when the line was busy, all of the test rings of the jacks of this 
line will be raised to a certain potential above that of the earth by 
virtue of being connected to the live side of the battery through the 
third strand of the cord circuit and third contact of the plug. As 
a result of this raising of potential of the test rings, the operator 
will get a click in her head telephone when she applies the test plug,, 
due to the flow of the current from the test ring through the tip 
of her plug to ground through a portion of the repeating coil. This 
alters the potential across the operator's receiver circuit, thus giving 
a click in the operator's ear. 

If the line is free she inserts the plug to its full extent and presses 
her ringing key, which rings the bell of the called subscriber. The 
insertion of the plug also operates the cut-off relay of the called 
line, thus cutting off its signaling apparatus. As soon as the call- 
ing plug is inserted in the jack of the called line the supervisory 
lamp associated with that plug will be illuminated, the same current 
serving to illuminate the lamp and to operate the cut-off relay. 
As soon, however, as the subscriber responds he will close the cir- 
cuit between the two sides of the line and thus allow current to pass 
through the line and furnish current to the transmitter. This cur- 
rent will pass through the supervisory relay, which will then be 
operated, and close the low resistance shunt about the lamp, thus 
extinguishing it. The answering supervisory lamp did not light at 
all when the operator answered the call, because the subscriber's 
telephone being off its hook, current at once passed through the 
answering supervisory relay, thus shunting the answering super- 
visory lamp. 

Both supervisory lamps, therefore, remain out during a conver- 
sation, but as soon as either subscriber hangs up his receiver the 
corresponding supervisory lamp will light, because the armature 
of the supervisory relay will fall back and allow the full strength 
of current to traverse the lamp. 

An important feature associated with the line-calling apparatus 
has been omitted from the circuits of the Western Electric Com- 
pany, so far considered. This has been done for the sake of sim- 
plicity in describing the main functions of the system. 

In order to afford additional means for attracting the attention 
of the operator when a call is made, what is termed a "pilot lamp" 
is placed on each position of the multiple switch-board, this lamp 
being so wired as to be lighted whenever any line lamp on that 
position is lighted in response to a call. This lamp (there being one 



320 



AMERICAN TELEPHONE PRACTICE. 



for every position) is located on a prominent portion in the face Oi 
the board, and is provided with a much larger lens than the line 
lamps, so as to be easily distinguishable, even to a person in a dis- 
tant part of the room. The circuit arrangement by which a single 
pilot lamp is associated with all of the lamps on a position, and by 
which a single night alarm-bell for the entire central office is associ- 
ated with all the pilot lamps, is shown in Fig. 267. 

Included in the common battery lead, which feeds all of the line 
lamps on any position, is placed a pilot relay, P, which is of low 
resistance so as to allow sufficient current to pass through it to 







tznz 



&g 



r- ^>^ AN&.JAC1\. 

«ilrtl LINE LAMP S.RELAT 

"ini INDIVIDUAL. TO 
S^E./VCH LINE. 
f| |\^*VjTO OTHER LINE LAMPS 

— SAW *■ IN SAME POSITION. 



MULT JACKS. 



PILOT RELAT 4. CAMP COMMON To ALU 
LINE. LAMPS IN SINGLE POSITION. 

\j/ C\S / ' ro PILOT LAMPS 

■TPP ty>>- J/^ POSITIONS. 



N. A. SWITCH 






-T5 



^\ 



^m O 



II5HT ALAUM RELA^f TO) CALLING C; E N E RATo R . 

COMMON TO ALL. PILOT 
LAMPS IN OFFICE „ o-+ 



FIG. 267.— WESTERN ELECTRIC LINE AND PILOT CIRCUITS. 



properly light several of the line lamps simultaneously. This relay 
is therefore in a portion of the circuit common to all of the line lamps 
on a position, and will therefore be operated whenever any line 
relay closes the circuit of any lamp on that position. The oper- 
ation of this pilot relay will light the pilot lamp, L, on that position. 
The 300-ohm resistance coil shown in multiple with the line lamp, S, 
in Fig. 267 is furnished, so that the subscriber, whose line lamp has 
burned out, can signal the exchange by lighting the pilot lamp. 

It is also customary, especially for use at night, to provide an 
alarm-bell, B, adapted to ring when a call is received on any line 
in the entire office. This is usually termed a "night alarm." For 
this purpose a single relay, N, is wired in the common portion of 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 321 

the circuit through which current flows from the battery to all of 
the line pilot lamps, this relay occupying the same relation to all 
of the pilot lamps in the office as each line pilot does to all of the 
line lamps on any one position. It therefore follows that when- 
ever any pilot lamp is lighted the common night-alarm relay will 
be actuated, which will close the circuit of the night-alarm bell. 
This bell may be an ordinary vibrating bell, but better practice is to 
make it a magneto bell, having it actuated by current from the regu- 
lar ringing generator. 

The common battery multiple switch-board of the Western Elec- 
tric Company, the circuits of which are shown in Figs. 264 to 267, 
is by no means the only one using three wires in the multiple, but 
it has the distinction of having been put into far greater use than 
any other common battery switch-board, probably greater than all 
other systems combined. It represents the highest development 
of the three-wire system, and the fact that it has been the standard 
system of the Bell companies for a number of years speaks for its 
efficiency. 

It will be seen that in this system the circuit is carried through 
the jacks to the third conductor of the cord circuits for the purpose 
of securing a reliable busy test and for securing the operation of- 
the cut-off relay and the supervisory apparatus. In this way all 
of these functions are performed by circuits separate from those 
used in the actual transmission of speech. In the design of the 
two-wire multiple switch-board, of which brief mention has already 
been made, a most difficult problem had to be faced at the outset. 
Since two wires or two conductors were absolutely necessary for 
talking purposes, it was necessary that means be devised by which 
all of these results could be accomplished over the same set of con- 
ductors, or such portion of them as were used for talking, and all 
this had to be accomplished without in any way interfering with the 
talking efficiency, which, after all, is of paramount importance. The 
problem is, however, infinitely more complex when applied to the 
modern common-battery system, because of the necessity of pro- 
viding for the automatic operation of both line and supervisory 
signals, and the obtaining of a reliable test without having any of 
these functions interfere with each other or with the proper supply 
of transmitter current from the central office battery to the sub- 
scribers. The first practicable common battery two-wire multiple 
switch-board svstem to come into extended commercial use is the 



322 



AMERICAN TELEPHONE PRACTICE. 



one developed by the engineers of the Kellogg Switch-board and 
Supply Company and put into extensive use by that company. 

The line circuit, which has been the standard circuit used in the 
Kellogg system for several years, is shown in Fig. 268. At the 
left of this figure is shown the typical, simplified sub-station appa- 
ratus, the sub-station circuit, actually used in the Kellogg sys- 
tem, having already been described in Chapter XIX. The two limbs 
of the line extend directly to two spring levers of the cut-off relay. 
Each of these springs normally rests against a back contact point 
when the relay is not energized, each being adapted to break this 
contact and make contact with another normally open point when 
the relay is energized. The back contact of that relay spring, which 
is connected with the sleeve side of the line, is connected through 
the coil of the line relay and the common battery to the ground. 




iifl 



AN5.JACK. 



SUB'S. 
STATION 



FIG. 268.— KELLOGG MULTIPLE SWITCH-BOARD LINE CIRCUIT. 



The back contact of the spring connected with the tip side of 
the line is connected directly to ground. With the cut-off relay 
in its normal position, therefore, the line relay is placed under the 
control of the subscriber in the same manner as in the line circuit 
of the Western Electric Company. This line relay controls a lamp 
placed adjacent to the answering jack. 

When the cut-off relay is energized the spring connected with 
the tip side of the line makes engagement with a contact connected 
to a wire leading to all of the tip springs of the answering and mul- 
tiple jacks belonging to that line, while the spring in connection 
with the sleeve side of the line engages a contact connected with 
all of the sleeve contacts of the jacks. The jacks have but two 
contacts, a tip and a sleeve, the sleeve forming the test ring as well 
as one terminal of the talking circuit. The coil of the cut-off relay 
is permanently connected between the sleeve contacts of the jacks 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 



323 



and the earth, so that the circuit over which it is operated embraces 
a portion of the talking circuit. 

Normally, when no plug is inserted into a jack of a line, the cut- 
off relay is not energized and the jacks are entirely disconnected 
from the line, while the signaling apparatus is operatively connected 
with the line. When a cut-off relay is operated by the insertion 
of a plug into a jack the line relay and line-signaling apparatus is 
cut completely clear of the line by the breaking of the back con- 
tacts on the cut-off relay, while the jacks are connected with the 
line by the making of the front contacts. 

In Fig. 269 is shown, stripped of some details for the sake of 
clearness, a cord circuit commonly used in connection with switch- 
boards employing the line circuits just shown. This circuit em- 



O 



££ 



inJ ran qn 



__.1L lf£ 



R3 






0£) 



R 2 



M ,JJ 



1 



¥£ 



R 4 




FIG. 269.— KELLOGG MULTIPLE SWITCH-BOARD CORD CIRCUIT 
SIMPLIFIED. 

ploys two-conductor plugs and two-conductor cords, the answering 
plug being shown at the left and the calling at the right. It will 
be seen that the tip of the answering plug is connected with the tip 
of the calling plug through a condenser, the sleeve of the answer- 
ing plug being connected to that of the calling plug through an- 
other condenser. It is through these condensers that the fluctu- 
ating voice currents pass during a conversation between two sub- 
scribers. Two batteries, 1 and 2, are employed, each having the 
positive pole grounded. The grounded pole of battery, i. is con- 
nected to the tip side of the answering cord through the winding 
of relay, R ly while the ungrounded pole of this battery is connected 
to the sleeve side of the same cord through the relay. R. 2 . It is 
through the windings of these relays that current is fed from "bat- 
tery, 1, for use in talking on the line to which the answering plug 



324 AMERICAN TELEPHONE PRACTICE. 

is connected. In the same manner the relays, R 3 and R±, connect 
the poles of battery, 2, with the tip and sleeve strands, respectively, 
of the calling plug, thus supplying current to the line with which 
this latter plug is connected. 

The relays, R t and R 2 , control the circuit of the supervisory lamp, 
Sa, while the relays, R 3 and R±, similarly control the circuit of the 
lamp, Sc. The relays, R x and R 3 , have normally closed contacts in 
the circuits of their lamps, while relays, R 2 and R±, normally hold 
their contacts open, these contacts also being in the circuits of their 
respective lamps. 

Considering now the relays on the answering side of the cord cir- 
cuit only, it is quite evident that the relay, R 2 , which is connected 
from the sleeve side to battery, will be actuated as soon as the an- 
swering plug is inserted into the jack, the circuit over which the 
current flows being traced from the battery through relay, R 2 , the 
sleeve strand of the cord circuit to the sleeve side of the jack, thence 
to ground through the cut-off relay and to the opposite pole of the 
battery. This current also serves to operate the cut-off relay, and 
both this relay and the relay, R 2 , remain closed as long as the plug 
is in the jack of the line, regardless of the position of the subscriber's 
switch-hook, and, in fact, regardless of any other changes. 

The operation of the relay, R 2 , will cause the lighting of the super- 
visory lamp, Sa, provided the relay, R lf is not actuated at the same 
time. It will be seen that if the subscriber on the line to which 
the connection is made has his receiver off its hook the relay, R lf 
will be operated over the following path : from the ungrounded pole 
of battery, I, through the relay, R 2 , to the sleeve side of the line to the 
subscriber's station and through his talking apparatus back to the 
tip side of the line, thence through the tip side of the cord circuit 
to the winding of relay, R t , to the grounded pole of the battery. 
The completeness of this circuit depends on whether the subscriber's 
receiver is on or off its hook, and therefore the relay, R lf is under 
the control of the subscriber. Since, when a connection is made 
with a line the relay, R 2 , is always operated, the only thing needed 
to control the lamp is the operation of the relay, R ± . From this it 
follows that when the connection is made with the line the lamp is 
under the control of the subscriber on that line. 

Instead, therefore, of making the supervisory lamp ready for 
operation by completing its normally open connection at the third 
contact of the plug and jack, as in the Western Electric system, this 
is done in the Kellogg system by a separate relay, which relay is 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 325 



operated by the insertion of a plug into the jack. This being the 
case it will be seen that the relay, R', of the Kellogg system corre- 
sponds exactly in function with the supervisory relay of the Bell 
system. 

The terminals of the calling generator are associated with the 
ringing key, so as to ring a subscriber in the ordinary manner. A 
little consideration will show, however, that if no other than the 
usual means are provided, the operation of a ringing key would, by 
cutting off the other portions of the cord circuit, cut off all battery 
from the line, and thus allow the de-energization of the cut-off relay. 
This would then be under the influence of the ringing cord, with the 
result that its armature would be rattled back and forth, alternately 
connecting and disconnecting the jacks from the line. In order to 
prevent this the back contact of the ringing key on the sleeve side is 




TO BATTTtinY. 



FIG. 270.-COMPLETE KELLOGG CORD CIRCUIT. 

connected with the ungrounded terminal of the common battery, so 
that, although the original connection with the battery is broken by 
the operation of ringing, it is at once re-established by the back con- 
tact of this key when thrown in the ringing position. A flow of di- 
rect current thus takes place from battery, through the back sleeve 
contact of the ringing key, and thence through the cut-off relay to 
ground. This serves to hold the cut-off relay in its operated position 
during ringing. A resistance coil of about 50 ohms is placed in the 
lead from battery to the back contact of the ringing key on the sleeve 
side, to prevent possible damage due to excess current should a 
ground occur in some of the key springs. 

In order to simplify the explanation of the supervisory' circuits 
some of the features by which a suitable test is made for "busy" lines 
were omitted from the cord circuit shown in Fig. 26Q. In Fig. 270 



326 AMERICAN TELEPHONE PRACTICE. 

these features have been added. It will be noticed that in the latter 
figure the relay, R 4 , is provided with an armature, a, which was not 
shown in Fig. 269, this armature serving normally, or when the 
relay is not operated, to hold' the tip side of the calling cord circuit 
open. In this position the tip of the calling plug, instead of being 
connected with the tip of the answering plug, as is necessary for 
conversation, is connected to ground through the test relay, T. 

The test relay, when operated, simply closes a circuit including 
an extra winding on the operator's induction coil, and battery No. 2, 
thus, by induction, causing a current to flow through the operator's 
head receiver and causing, the customary click. 

The operation of the test system may now be described : When 
a line is not busy each of the test rings or sleeve contacts of the jacks 
belonging to that line is maintained at the same potential as the earth, 
these contacts being connected to no source of electromotive force 
and being connected to earth through the coil of the cut-off relay. 
If, under these circumstances, the tip of the calling plug is applied to 
the test contact of the jack no effect will be produced on the test 
relay, because there will be no source of electromotive force in the 
circuit thus closed, and the relay will not respond. Moreover, there 
will be no chance of the test rings being raised to a potential above 
that of the earth from some extraneous source, as, for instance, a 
cross on the line, because these test rings are entirely removed from 
connection with the external lines, being separated from them at 
the cut-off relay. 

If, however, the line tested is busy by virtue of being connected 
to at some other section, all of the test rings belonging to that line 
will be raised to a certain potential above the earth, since they will 
be connected directly with the ungrounded side of the battery 
through the sleeve contact of the plug. When a test of a jack of a 
busy line is made at any position, current will flow from the test 
ring through the tip of the calling plug used in making the test, to 
the armature, a, of the relay, R*, thence through the back of this 
armature to the coil of the test relay, thence to ground. The current 
thus flowing will energize the test relay, and by closing its local cir- 
cuit through the extra winding on the operator's induction coil, will 
cause a click in the operator's telephone. In order for this test to be 
operative the relay, T, must be quick-acting, so as to respond to the 
slightest touch between the tip of the calling plug and the ring on the 
jack being tested. 

The line pilot and night alarm circuits have been omitted from the 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 



327 



figure illustrating- the Kellogg system. They are associated with the* 
line lamp circuit in practically the same manner as in the Western 
Electric system. The operation of these will, therefore, be made 
clear from the description of the Western Electric pilot circuit 
already given in connection with Fig. 267. 

The Kellogg, Company also applies the pilot feature to the super- 
visory signals in such a manner that a pilot lamp will light on any 
position whenever the subscriber who originated the call hangs up 
his receiver. In other words, the pilot lamp will light whenever the 
subscriber to whose line the answering plug is connected hangs up 
his receiver. The night alarm bell will also sound if the switch con- 
trolling it is in the proper position. This is readily accomplished by 
placing in series with the supervisory lamp associated with the an- 
swering plug a pilot relay which controls the pilot lamp for that 
position, it being understood, of course, that this lamp is common 




Sub'S . 
STATION 



FIG. 271. 



-STROMBERG-CARLSON TWO-WIRE MULTIPLE SWITCH-BOARD 
LINE CIRCUIT. 



to the circuits of all the answering supervisory lamps on that 
position. 

In a common portion of the circuit belonging to all of the super- 
visory pilot lamps in the office is placed a night alarm pilot relay in 
the same manner as the night alarm relay was applied to all the line 
pilot lamps. This supervisory night alarm is useful at night for call- 
ing the attention of the operator to the fact that a disconnection is 
desired on some portion of the board. 

The Stromberg-Carlson Company during the last few years has 
installed in a number of places a two-wire multiple common battery 
system, the line circuit of which is shown in Fig. 271. In this cir- 
cuit the cut-off relay, C, is provided with two coils differentially 
wound, so that when the two windings are traversed by equal cur- 
rents in opposite directions no effect will be produced upon the core. 
These windings are put on the core "in tandem," so to speak, one 



328 AMERICAN TELEPHONE PRACTICE. 

winding being on each end, and in order to have the rear winding as 
effective as the one nearest to the armature the former is given a few 
more turns, and, therefore, a slightly greater resistance. Included 
between the windings and in series with them is the coil of the line 
relay, L, and a 40-volt storage battery, this latter being common to 
all lines. 

The tip side of the line is connected to one terminal of one of the 
windings of the cut-off relay, and also to the tip springs of all the 
jacks of that line. The sleeve side of the line is connected to the arm- 
ature of the cut-off relay, the back contact of which is connected 
with one terminal of the second coil of the cut-off relay. Under nor- 
mal conditions, therefore, that is, when the cut-off relay is not ener- 
gized, the two windings of the cut-off relay, together with that of the 
line relay and the common battery are all connected in series across 
the circuit of the line. It will be evident that when a subscriber's re- 
ceiver is removed from the hook, thus closing the circuit of the line 
at the sub-station, current will flow through all of these relay coils 
operating the line relay and illuminating the line lamp. The cut-off 
relay will not, however, be operated because of its differential wind- 
ing. 

The front contact of the cut-off relay is normally open, but is per- 
manently connected to all the sleeve contacts of the jacks of the line. 
These latter contacts are, therefore, normally disconnected from the 
line, but are connected therewith when the cut-off relay is actuated. 

As will be seen from the discussion of the next figure the cut-off 
relay will be operated when the operator plugs into a jack in response 
to a call, this act completing a circuit through one of the coils of the 
cut-off relay and causing it to attract its armature. The attraction 
of the armature of the cut-off relay by breaking the back contact, 
opens the circuit from the battery through the line relay and the 
front coil of the cut-off relay, thus de-energizing both of these coils. 
The line lamp is thus extinguished, and it is evident that the line 
relay cannot be again operated until the cut-off relay is de-energized. 

While this line circuit has been used almost exclusively in the com- 
mon battery work of the Stromberg-Carlson Company for several 
years past, the cord circuit used with it has been subject to many 
changes. 

In Fig. 2.72 is shown one of these cord circuits, connecting two 
of the line circuits shown in Fig. 271. In this cord circuit the 
answering and calling plug are of the two-conductor type and are 
connected by two-conductor cords, as in the Kellogg system. The 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 



329 



sleeve side of the circuit of each plug contains a supervisory relay, 
each of these relays being permanently shunted by a two-microfarad 
condenser in order to prevent the retardation of the relay coils ffom 




interfering with the transmission of the voice currents through the 
sleeve side of the cord circuit. The relay, S s , has its coil connected 
between the positive side of the battery and the tip of the answering 
plug under normal circumstances. This relay, when operated, serves 



330 AMERICAN TELEPHONE PRACTICE. 

to connect by one of its armatures the positive side of the battery 
with the local circuits of the supervisory lamps and by its other 
armature to break the normally closed test circuit and close the talk- 
ing circuit between the answering and calling contacts of the plugs. 

The operation of this system may now be understood. Assuming 
the subscriber of the left hand line to be making a call, his line lamp, 
lights in response to the removal of his receiver from its hook, as 
already described. In response to this signal the operator inserts the 
answering plug into the answering jack, which, beside completing 
the talking circuit between the line and the cord circuit also estab- 
lishes the following circuit which includes one coil of the cut-off 
relay. This circuit may be traced from the positive side of the bat- 
tery through the winding of the relay, S 3 , to the tip of the answering 
plug, thence over the tip side of the line through the rear winding 
of the cut-off relay to the negative side of the battery. This un- 
balances the cut-off relay and causes its operation, which, as before 
stated, causes the de-energization of the line relay and the extin- 
guishing of the line lamp. The operation of the cut-off relay also 
completes the connection between the sleeve contacts of the jacks 
and the sleeve side of the line, whch connection had hitherto been 
open at the cut-off relay. 

The same current which thus causes the initial operation of the 
cut-off relay also causes the operation of the relay, S 3 , which there- 
fore attracts both of its contact levers. The attraction of the left 
hand lever of this relay completes the connection between the posi- 
tive side of the battery and one side of the circuit of both super- 
visory lamps, so that with this relay actuated, the lighting of the 
supervisory lamps depends entirely on the condition of the relays, 
S x and S 2 . If either of these relays is de-energized while the relay. 
S 3 ,\s energized the corresponding lamp will be lighted. It will thus 
be seen that the relay, S 3t of this system, so far as its left-hand lever 
is concerned, corresponds in function with the relays, R 2 and R 4 , of 
the Kellogg system, as shown in Fig. 270, in that it is operated when 
the plug is inserted into a jack, and that when so operated the 
supervisory lamps are placed under the exclusive control of their 
supervisory relays. 

Current for talking purposes is now fed to the left-hand line from 
the positive pole of the battery through the impedance coil, 7, to the 
sleeve side of the line, and from the negative pole of the battery 
through the rear winding of the cut-off relay, C, to the tip side of 
the line. This current flows only when the subscriber's receiver is 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 331 

removed from its hook and therefore the supervisory relay, S lf 
which lies in the path of this current, is under the control of that 
subscriber. When the operator plugged into the jack in response to 
a call, therefore, the supervisory relay, S lf was operated, thus pre- 
venting the lighting of the answering supervisory lamp. 

In order to communicate with a subscriber the operator throws 
her listening key, which connects the talking circuit with the line in 
the usual manner, and at the same time cuts a condenser, K, into the 
tip side of the cord circuit. The purpose of this condenser will be 
pointed out later. 

In order to test for a busy line the operator will apply the tip of 
the calling plug to the test ring of the jack in the usual way. As in 
the Kellogg system, the tip side of the calling cord circuit is nor- 
mally broken ; in this case, by the right-hand contact lever of the 
relay, S 3 . The tip of the calling plug is by the back contact of this 
lever normally connected to the negative side of the battery through 
an extra winding on the operator's induction coil. Instead of using 
an extra test relay, as in the Kellogg system, the test circuit is run 
directly through one winding of the induction coil, so as to throw 
the test current directly into the operator's coil, rather than doing it 
through a local circuit controlled by a relay. If the line tested is 
free its test rings will be disconnected from the line at the cut-off 
relay, and they will have no source of electromotive force upon them. 
When, therefore, the test is made with such a line, no current will 
flow, and the operator receiving no click will know that the line is 
free. If, however, the line is busy, the test rings will be in electrical 
connection with the positive side of the battery through the sleeve 
strand of the cord connected with the line at another section. Under 
these conditions current will flow from the test ring at the section 
where the test is made through the extra winding on the induction 
coil to the negative side of the battery, giving the operator a click in 
the ear. 

It will be noticed that the test can only be made when the relay, 5* 8J 
is de-energized, as otherwise the extra winding in the induction coil 
would be disconnected from the tip of the calling plug. Since the 
relay, S s , was energized when the operator plugged in with the 
answering plug, it becomes necessary in making a test to de-energize 
this relay temporarily, and this is the function of the condenser. A', 
already referred to. This condenser is interposed in the tip side of 
the cord circuit when a test is made so as to prevent direct current 
from flowing through the winding of the coil, S* s , thus allowing the 



332 AMERICAN TELEPHONE PRACTICE. 

contact levers of this relay to fall back into proper position for test- 
ing. As the throwing of the listening key must necessarily be per- 
formed before a test can be made, the de-energization of the relay, S 3 , 
is brought about without additional effort on the part of the oper- 
ator. The temporary cutting-off of the current through the relay, 
S 3 , while listening in, does not de-energize the relay, C, because the 
current for talking purposes flows through one of the coils of this 
relay from the negative side of the battery, through the tip of the 
cord, the primary winding of the operator's induction coil, the 
transmitter, the windings of relay, S x , and retardation coil, /, to the 
positive side of battery. 

Having learned that the line is free, the operator inserts the call- 
ing plug, into the multiple jack tested, and by means of a ringing key 
calls the subscriber in the usual manner. After a connection is thus 
completed and the tw r o subscribers are conversing, the act of listen- 
ing in by the operator will not de-energize the relay, S 3i because that 
relay w r ill still receive current, which passes through it and the pri- 
mary of the operator's telephone set. The presence of the condenser, 
K, when interposed in the tip side of the talking circuit during a con- 
versation between two subscribers by the act of the operator's lis- 
tening in does not, of course, prevent conversation between those 
subscribers. After the called subscriber has answered the relay, S 3 , 
remains energized as long as the connection is established, and there- 
fore both of the relays, S t and S 2 , are under control of their re- 
spective subscribers. As soon as either of the subscribers hangs up 
his receiver the corresponding relay will be de-energized, thus light- 
ing the corresponding supervisory lamp and conveying the super- 
visory signal to the operator in the usual manner. 

In establishing the connection, as soon as the operator answers by 
plugging in with the answering plug, the calling supervisory lamp 
w r ill be lighted, because the relay, S 3 , will be operated, while the re- 
lay, S 2 , receiving no current, will remain inert. This lamp should 
not light until the operator had plugged into the jack of the called 
subscriber, as is the case in the standard code of supervisory signal- 
ing. This premature lighting of the calling supervisory lamp, how- 
ever, probably does little harm, except that it tends to slightly con- 
fuse the operator in interpreting the supervisory signals, and also 
causes a slightly greater consumption of current than is actually nec- 
essary. 

In order to prevent the operation of the ringing key from discon- 
necting the battery from the called line during the ringing of a sub- 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 333 

scriber, and thus allowing the ringing current to rattle the armature 
of the cut-off relay of that line, the tip side of the ringing key, which 
carries one terminal of the generator, is also connected to the posi- 
tive terminal of the battery, so that battery current is thrown onto 




the calling line during the ringing for the purpose of keeping the 
cut-ofl relay constantly energized, this feature being almost the 
same as that described in the Kellogg system. 

The Stromberg-Carlson Company has recently adopted a three- 



334 AMERICAN TELEPHONE PRACTICE. 

wire system which it is now installing in the several large offices in 
the new Kansas City, Missouri, exchange. The line and cord cir- 
cuits of this system are shown in Fig. 273. 

The line circuit is very similar to that of their two-wire system, 
the cut-off relay being differentially wound and normally connected 
across the line in series with the line relay and the common battery. 
This circuit through the line relay and battery is broken at the back 
contact of the cut-off relay when the latter relay is operated. In- 
stead of having the test rings connected directly with the sleeve 
spring of each jack, a third wire runs throughout the multiple jacks, 
which is connected with a test ring at each section. This wire ter- 
minates in the front contact of the cut-off relay in such a manner that 
when this relay is operated all of the test rings will be connected 
directly with the sleeve side of the line. Under normal circum- 
stances, however, all of the test rings belonging to the line are insu- 
lated from ground and from all other portions of the system. The 
sleeve springs in the jacks are permanently connected with the sleeve 
side of the line. 

Each side of the cord circuit is provided with two relays, one, A or 
A\ adapted to be energized when the operator plugs into a line jack, 
thus closing a normally open point in the circuit of the correspond- 
ing supervisory lamp. The other relay, B or B' , is in the path over 
which current is fed to a subscriber's line for talking, and is thus 
placed under the control of the subscriber with whose line the cor- 
responding plug is connected. This relay, after the first relay has 
been operated, absolutely controls the supervisory lamp. 

When the operator plugs in in response to a call, current at once 
flows from the ungrounded pole of the battery through the coil of the 
relay, A, to the tip side of the line and to ground through one coil 
of the cut-off relay. This operates the cut-off relay, causing it to 
cut off the line relay and at the same time to connect the test rings 
of the line with the sleeve side of the line. Current is fed to the 
subscriber's instrument from the ungrounded pole of the battery 
through the coil of the relay, B, to the sleeve side of the line, and 
from the grounded pole of the battery through one coil of the cut-off 
relay to the tip side of the line. 

The test, as is usual, depends upon the condition of the test rings 
as governed by the insertion of a plug in a jack at another section. 
If the line is free the test rings will be insulated from everything 
except the other test ring on that line, while, if the line is busy, 
they will be connected to the negative pole of the battery through the 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 335 

coil of the relay, B, or B', belonging to the cord connected with the 
line at another position. The test rings of the busy line will there- 
fore be raised to a potential above that of the earth. The listening 
key, L, is provided with the ordinary contacts for connecting the 
operator's telephone, and with an auxiliary pair of contacts for con- 
necting the tip of the calling plug to ground through a test relay 
when the listening key is operated. The relay, A\ corresponding 
with the relay, A, on the answering side carries an additional con- 
tact lever, a, which, when operated, by the insertion of a plug in a 
jack closes the tip side of the calling cord around the contacts on the 
listening key in such manner that the listening key, when operated, 
will not disturb the continuity of the tip conductor. Until the relay, 
A', is operated, however, the operation of the listening key inter- 
rupts the continuity of the tip conductor, and at the same time con- 
nects the tip of the calling plug to ground, through the testing ap- 
paratus. 

In testing a busy line with the listening key, L, thrown into the 
listening position, current will flow from the test ring through the tip 
contact of the calling plug to ground through the test relay, thus 
giving the operator a click in the ear in the same manner as with 
the test relay in the Kellogg system. Under this condition the 
throwing of the listening key opens the tip strand of the cord circuit. 
When the calling plug is inserted into a jack, however, the subse- 
quent operation of the listening key on the part of the operator will 
not interrupt the continuity of the tip conductor, because, owing to 
the operation of the relay, A', the open contact in the listening key 
will be shunted by the armature, a, and its contact. The operation 
of ringing is the same as has already been described in connection 
with other circuits. 

The supervisory signals are controlled in practically the same 
manner as those of the Kellogg system, the relays, A and A', clos- 
ing the normally open contacts in their respective lamp circuits when 
their plugs are inserted into the jacks of the connected lines, after 
which the current, flowing through the relays, B and B', to the sub- 
scribers' stations, operates these relays to prevent illumination of 
the corresponding lamps as long as the lines are in use. As soon, 
however, as either subscriber hangs up his receiver the current 
from the relay, B or B 1 ', ceases to flow, thus allowing the armature 
to drop back, lighting the lamp. 

Of considerable interest is the circuit of the Sterling Electric Com- 
pany, of La Fayette, Ind., shown in Fig. 274. This is a three-wire 



33G 



AM ERIC AX TELEPHONE PRACTICE. 



multiple system and has the distinction of possessing the simplest 
cord circuit of any common battery multiple system in existence. 
Moreover, there is but one relay for each line, this relay serving not 
only as a line relay, but also as a supervisory relay after a connec- 
tion has been made. 

In this the line relay has two active windings, which, however, 
are not differentially wound. One of these windings is connected 
between one side of the battery and one side of the line, the other 
winding being similarly placed with regard to the other side of the 
battery and line. As this relay is permanently connected with the 




FIG. 274.— CIRCUITS OF STERLING MULTIPLE SWITCH-BOARD. 



line, it is always under the control of the subscriber, the current 
for talking purposes being fed through the two windings of this 
relay to the line, this being the only connection at any time between 
the line and the battery. 

When a subscriber removes his receiver from the hook for the 
purpose of making a call the relay is operated, thus closing the cir- 
cuit of the line lamp, which circuit also includes a no-ohm resistance 
coil. The line lamp is adapted to 10 volts, and is thus brought to 
its full candle power when subjected to 24 volts through the exter- 
nal resistance of no ohms. When the operator inserts the answer- 
ing plug connection is made between the tip and sleeve side of 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 337 



the line and the corresponding conductors of the cord circuit, and 
also the third strand of the cord is connected with the third con- 
ductor of the jacks, this conductor being connected with the test 
rings of the jacks and with the contact lever of the line relay. The 
third strand of the cord includes the supervisory lamp, which is 
adapted to six volts. The insertion of the plug does not de-ener- 
gize the line relay, which still attracts its armature, but it extin- 
guishes the line lamp by placing in shunt with it the 6-volt super- 
visory lamp of the cord circuit. In this condition, while both lamps 
receive some current, neither is illuminated because of the presence 
of the no-ohm resistance with which the two shunted lamps are 









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FIG. 275.— DETAILS OF STERLING CIRCUIT. 

in series. The circuit of the line and supervisory lamp under this 
condition is shown diagrammatically in Fig. 275. 

The system is such that a line will test ''busy" as soon as a sub- 
scriber removes his receiver from the hook, or, in other words, as 
soon as the line relay is operated. As will be seen, the test rings 
are normally connected to earth in such a manner as to include no 
source of potential. As soon, however, as the line relay is oper- 
ated they are connected through the line lamp with the ungrounded 
terminal of the common battery, thus raising their potential above 
that of the earth. The same result also occurs, regardless of whether 
the subscriber's receiver is on or off its hook, when the operator 
plugs into the jack of any line, for then the potential of the test 
rings on that line are raised by being connected to battery through 
the supervisory lamp and the third strand of the cord. When, there- 
22 



338 AMERICAN TELEPHONE PRACTICE. 

fore, an operator tests by applying the tip of the calling plug to the 
test ring of a line, she will get a click if a subscriber has removed his 
receiver from the hook or if the line is connected with at another 
section, while she will get no click if the line is free. 

When either subscriber hangs up his receiver the corresponding 
line relay will be de-energized, thus allowing its armature to fall 
back. This removes the shunt of the io-volt lamp from about the 
6-volt supervisory lamp, and at the same time puts a low resistance 
shunt (39 ohms) about the no-ohm resistance coil. The circuit 
through the supervisory lamp is then changed from that shown in 
Fig. 275 to that shown in Fig. 276. The supervisory lamp is not 
shunted itself, while it has in series with it the combined resistance 




FIG. 276.— DETAILS OF STERLING CIRCUIT. 

of the no-ohm and the 39-ohm coil in parallel, under which con- 
ditions this lamp is lighted to its full candle power, thus giving to 
the operator the supervisory signal. 

In order to separate the two sides of the cord circuits from each 
other, and thus prevent the signals of one line interfering with those 
of another line connected with it, two condensers are used in each 
cord circuit, one being placed in each strand of the talking circuit. 

In practice, both of the resistance coils used in connection with 
the line and supervisory lamp are wound on the core of the line re- 
lay, thus giving this relay in all four windings, two of which, how- 
ever, are in the nature of dead resistance. 

This system has one very objectionable feature in that it is im- 
possible, without extremely complex arrangements, to provide for 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 339 

either line or supervisory pilot lamps, and the advantages of these 
lamps, which are unquestionably great, are not attained in practice. 

Another disadvantage inherent in such a system is that of the 
difficulty of obtaining the proper uniform illumination of the lamps 
for line and supervisory purposes. Switch-board lamps when newly 
made are not as uniform with respect to voltage and candle power 
as are the larger lamps used for general illuminating purposes. 
Moreover, they change with age even more rapidly than larger 
lamps. When, therefore, the lamp must necessarily work in circuit 
with a considerable external resistance, and when the illumination 
or non-illumination of the lamp is dependent on certain changes in 
the amount of this external resistance; in other words, when the 
lamp is governed entirely by the different arrangements of certain 
fixed resistances in its circuit, a considerable amount of difficulty 
is experienced in properly maintaining the required illumination or 
non-illumination under the conditions of practice. This would not 
be the case were the lamps of uniform resistance and candle power, 
or if the voltage of the battery supplying the current were constant. 
As is well known, however, 4 a storage battery cell may vary in its 
voltage, according to the state of charge, from 2.5 volts down to 
1.7. Here we have (under extreme conditions) a variation of ap- 
proximately 40 per cent, with respect to the normal working press- 
ure, usually assumed to be 2 volts. It is, of course, possible to 
keep the voltage of the storage cell between much narrower limits 
than this, which would reduce this percentage of fluctuation by half. 
Inasmuch as the two resistance coils associated with the line relay 
must of necessity be fixed in their resistance value, such a circuit 
brings us in practice against the difficulty of maintaining a rather 
close adjustment between the circuit arrangements of these resist- 
ances with respect to the lamps and the voltage of the battery when 
both the condition of the lamps and the voltage of the battery are 
subject to variation. The difficulty in this respect is enhanced by 
the fact that after an exchange has been in operation some time new 
lamps have to be added to replace those burned out which gives 
a condition wherein lamps of various ages must be used under the 
same conditions. Such considerations as this have not, however, 
prevented such systems from being successful in practice. 

The line and cord circuits of the North Electric Company, of 
Cleveland, Ohio, is shown in Fig. 277. The arrangement of the 
line relay in this circuit is not unlike that of the Sterling circuit 
just described, this relay having a split winding, including between 



340 



AMERICAN TELEPHONE PRACTICE. 



them the common battery, the two windings and the battery in series 
being connected permanently across the metallic circuit of the line. 




FIG. 277.— NORTH ELECTRIC COMPANY'S MULTIPLE SWITCH-BOARD 

CIRCUIT. 

The third contact on the jack is wired through the coil of a cut-off 
relay to ground, the arrangement in this case beig similar to that 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 341 

employed by the Western Electric circuit. The cut-off relay, how- 
ever, in the North circuit, instead of serving to separate the line 
relay and signaling apparatus entirely from the line, simply serves 
to open the circuit of the line lamp, this circuit being controlled 
at two places, one by a normally open pair of contacts in the line 
relay, and the other by a normally closed pair of contacts in the 
cut-off relay. 

The cord circuit has three-conductor cords and three-contact 
plugs, two batteries being used as in the Kellogg system. Current 
is supplied from battery, A, to the two talking strands of the an- 
swering cord through the two windings, respectively, of the relay, 
R, current being similarly supplied to the talking strands of the 
calling cord through the two windings of the relay, R'. These two 
relays which control the supervisory lamps are not differentially 
wound, their windings both operating in the same direction to ener- 
gize their cores. The third strand of each cord circuit has two 
branches, controlled by the armature of the corresponding relay, 
R. One of these branches leads, when the relay armature is not 
energized from the ungrounded pole of the battery through the 
supervisory lamp in multiple with resistance coils, to the third con- 
tact of the plug. Under this condition, the lamp is shunted by two 
resistance coils in series, having a combined resistance of about 
500 ohms, which is too high a shunt to prevent the lamp from light- 
ing. When the armature of the relay, R or R', is attracted the cir- 
cuit of each lamp is opened, the circuit of the third strand then lead- 
ing from the ungrounded pole of the battery through a 300-ohm 
resistance coil alone to the third contact on the plug. When a plug 
is inserted into a jack the circuit of the third strand is completed 
by means of the test ring of the jack through the coil of the cut-off 
relay to ground. With the circuit so established the cut-off relay is 
operated and the lighting of the supervisory lamp depends on the 
non-attraction of the armature of the supervisory relay. As will be 
seen, the energization of the supervisory relay depends on the sub- 
scriber having his receiver off the hook for conversation, and there- 
fore the supervisory lamp is placed under control of the subscriber 
with whose line the corresponding plug is connected. 

When a subscriber calls, the line relay is energized by the cur- 
rent flowing through his instrument, thus lighting the line lamp 
When the operator inserts the answering plug the cut-off relay is 
energized, which performs no other function than to open the cir- 
cuit of the line lamp, leaving the line relay still energized. The line 



342 



AMERICAN TELEPHONE PRACTICE. 



lamp is thus extinguished. An additional path is maintained for 
the flow of battery current to each side of the line, this being through 
the coils of the relay, R. As the coils of the relay, R, are of about 
115 ohms resistance each, while those of the line relay each have 
a resistance of about 300 ohms, a greater portion of the current flow- 
ing to line passes through the supervisory relay coils, and this relay 
is, of course, energized. Like the line relay, it remains energized as 
long as the subscriber's receiver is removed from its hook. The 
conditions so far described are exactly reproduced with respect to 
the calling plug and the line called for, and therefore, when a con- 
nection is made between two lines the talking circuit may be repre- 
sented as in Fig. 278. The current from battery, A, is supplied 
to the line of the calling subscriber through the coils of the line 



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FIG. 278.— SIMPLIFIED NORTH CIRCUIT. 



relay belonging to that line and the supervisory relay of the an- 
swering plug. Similarly, the current is supplied from battery, B, 
to the line of the called subscriber through the coils of the line 
relay belonging to that line and the supervisory relay of the calling 
plug. It will be seen that this resolves itself into the same scheme 
of supplying current to the subscriber as that shown in Fig. 232 
of a previous chapter. 

As in most of the systems already described, the test rings of a 
free line are connected to earth and are at the same potential as the 
earth. The test rings of a "busy" line are subjected to the poten- 
tial of the battery through the third strand of the cord used in mak- 
ing a connection with the line, and are therefore raised to a certain 
potential above the earth. If, therefore, when an operator tests 
by applying the tip of a calling plug to the test ring she will get no 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 343 

click if the line is free, and will therefore insert the plug and ring. 
If, however, the line is busy, current will flow from the tip of the 
calling plug used in making the test over the sleeve side of the line 
through one winding of the supervisory relay, R', and to ground. 
This will momentarily raise the potential of the tip side of the cord 
circuit, thus causing a flow of current through the operator's re- 
ceiver bridged across the cord circuit when the test is made. 

When the answering plug was inserted into the jack of the call- 
ing subscriber the answering supervisory lamp did not light because 
of the immediate energization of the relay, R, due to the subscriber 
having his receiver off its hook. When the calling plug was in- 
serted into the jack of the called subscriber, however, the calling 
supervisory lamp was at once lighted, remaining so until the sub- 
scriber responded, at which time the energization of the supervisory 
relay, R', extinguished this lamp. When either subscriber hangs 
up his receiver the corresponding supervisory relay will be de-ener- 
gized, thus lighting the supervisory lamp. 

The line and cord circuits of the International Telephone Manu- 
facturing Company, of Chicago, are shown in Fig. 279. In the 
line circuit of this system a 500-ohm line relay and a 100-ohm cut- 
off relay are connected in series with the battery between the tip 
and sleeve strands of the line, respectively, this connection being 
permanent. When a subscriber removes his receiver from the hook 
for the purpose of calling, the line relay only will respond, although 
the current flowing traverses the coils of both relays. This is be- 
cause the line relay is much more easily operated than the cut-off 
relay on account of its higher resistance and more delicate adjust- 
ment. The circuit of the line lamp is controlled by the armature 
of both the line and the cut-off relays, the cut-off relay having its 
contact normally closed, that of the line relay being normally open. 
When, therefore, the line relay only is operated, the line lamp will 
be illuminated. The cord circuit contains three relays, two of 
which, A and B> are connected between the negative side of the 
battery and the tip strands of the answering and calling plugs, 
respectively, the points of connection between these two relays and 
their respective tip strands being separated by a two-microfarad con- 
denser. The relay, C, is connected between the negative side of 
the battery and the sleeve strand of both the calling and answer- 
ing plugs, this latter strand being continuous from the sleeve of 
the answering plug to the sleeve of the calling plug. This latter 
relay serves to control the connection between the negative side of 




FIG. 279.-INTERNATIONAL MULTIPLE SWITCH-BOARD CIRCUIT. 

341 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 345 

the battery and both supervisory lamps, this contact being nor- 
mally open, but closed upon the operation of the relay. 

When the answering plug is inserted into the jack of a calling 
subscriber the cut-off relay of the line, and the relay, C, of the cord 
circuit are at once operated, the circuit being traced from the nega- 
tive side of the battery through the coil of the relay, C, thence over 
the sleeve strand of the answering cord to the sleeve contact of the 
jack, thence to the positive pole of the battery through the coil of the 
cut-off relay. The energization of the cut-off relay opens the cir- 
cuit of the line lamp, thus extinguishing it, and at the same time 
relay, C, completes the circuit between the negative side of the bat- 
tery and both of the supervisory lamps. The calling supervisory 
lamp will thus be lighted upon the insertion of the answering plug, 
because the relay, B, which controls the other side of the circuit 
of the calling supervisory lamp will not be energized. The answer- 
ing supervisory relay will not, however, be lighted, because the re- 
lay, A, will receive current, this current flowing from the negative 
side of the battery through the coil of this relay to the tip side of 
the cord circuit and the tip side of the line, thence to the subscriber's 
station and back to the positive side of the battery through the coil 
of the cut-off relay. This will therefore hold the circuit of the an- 
swering supervisory lamp open as long as the calling subscriber 
keeps his receiver off its hook. 

When the operator inserts the calling plug into the jack of the 
called subscriber, in case his line is found to be free, the calling 
supervisory lamp will remain lighted until the subscriber responds, 
at which time the energization of the relay, B, due to current flow- 
ing through the called subscriber's instrument, will open the cir- 
cuit of this lamp and cause its illumination to cease. When either 
subscriber hangs up his receiver the corresponding relay, A or B, 
will be de-energized, thus illuminating the corresponding lamp. Ex- 
cept for the premature illumination of the calling supervisory lamp, 
when the answering plug is inserted into the line of the calling sub- 
scriber, the operation of the supervisory signals is in accordance 
with standard practice. 

In a free line the test rings are supposed to be subjected to no 
potential from the battery, these rings being grounded through 
the coil of the cut-off relay. When, however, a line is busy by virtue 
of being connected to, the test rings will be raised to a potential 
above that of the earth on account of being connected to the 
ungrounded side of the battery through the supervisory relay. C s 



346 AMERICAN TELEPHONE PRACTICE. 

and the sleeve strand of the cord. In applying the tip of a calling 
plug for the purpose of a test, the listening key being, of course, 
thrown, currrent will flow from the test ring of the busy line through 
the tip of the calling plug and the auxiliary pair of springs, a, closed 
when the key is in its listening position, through the operator's 
telephone receiver and the secondary winding of her induction coil, 
and through the test coil, T, to ground. This will produce a click 
in the operator's receiver, whereas, if the line is not busy no such 
current will flow, the test circuit then being from ground to ground 
and the operator will know that the line is free. 

This test circuit would appear faulty, as, in fact, is almost any 
test circuit wherein the test rings are permanently connected with 
one side of the line. To illustrate : If in this system the sleeve side 
of the line is crossed with some circuit on the outside of the central 
office, and thereby subjected to an electromotive force, all the test 
rings on that line would be raised to a potential above that of the 
ground and the result would be that the line would test busy. If the 
two sides of the line were crossed, current would flow from the tip 
side of the line to the sleeve side of the line, thus raising the potential 
of all the test rings belonging to that line, under which condition 
it would again test busy. The same result would be produced in 
testing a line on which a subscriber had raised his receiver from the 
hook, but which line had not yet been connected to at the central 
office. This, however, would not prove an objection, as no harm 
would be done if such line did test busy. 

It may be said in general that practice has proved that the test 
rings should either be completely localized by means of a separate 
test ring and test wire, as in the Western Electric, North and Strom- 
berg-Carlson three-wire systems, or that the test rings should be 
entirely disconnected from the external line during the time when 
the line is free. Unless one or the other of these methods is fol- 
lowed, false "busy" tests are liable to result. 

In Fig. 280 are shown the line and cord circuits of the American 
Electric Telephone Company, of Chicago, which is the latest circuit 
used by this company which has been brought to the attention of the 
writer. 

In this the line relay, A, has one of its terminals permanently con- 
nected with the tip side of the line, the other terminal being grounded 
through a 175-ohm retardation coil, R. The sleeve side of the line 
is permanently connected through a 175-ohm retardation coil, R', to 
a 40-volt storage battery, the other terminal of which is grounded. 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 34- 



As a result the line relay is permanently bridged across the two sides 
of the line in series with the two retardation coils and battery, and 




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this relay will, therefore, be operated by the removal of the sub- 
scriber's receiver from its hook. 



348 



AMERICAN TELEPHONE PRACTICE. 



The auxiliary spring, s, in the jack, which makes contact with 
the corresponding sleeve on the plug is connected to that terminal 
of the line relay which is not connected to the tip side of the line. Be- 
tween the corresponding contact on the answering plug and the tip 




FIG. 281.— DETAILS OF AMERICAN ELECTRIC CIRCUIT. 

contact is permanently bridged the io-ohm coil of the relay S. As a 
result of these connections the line relay, A, is shunted by the io-ohm 
coil whenever a plug is inserted into a jack. 

The line circuit, simplified to illustrate more clearly the action 
when a subscriber is calling, is shown in Fig. 281. The condition 




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FIG. 282.-DETAILS OF AMERICAN ELECTRIC CIRCUIT. 



after the operator has inserted the plug, showing clearly the shunting 
of the line relay coil by the low resistance relay coil, S, is shown in 
Fig. 282. 

As a result of the placing of the shunt around the coil of the line 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 



54S 



relay, this latter relay is deprived of a sufficient amount of its current 
to cause it to drop back, thus putting out the lighted lamp. The re- 
lay, S, is thus energized as long as the subscriber's receiver is re- 
moved from its hook, this relay being in the path over which current 
is supplied to the subscriber, and by the operation of its armature it 
renders impossible the operation of the supervisory relay, B, on the 
answering side of the cord, since the circuit of this latter relay passes 
through the back contact of the relay, S. The arrangement so far 
treated is the same on the calling as on the answering side of the 
cord circuit, and therefore when a connection is made between the 
two lines the line relay of the called line is shunted out of service by 
the coil of the relay, S\ which latter relay is operated as soon as the 
subscriber removes his receiver from its hook, thus opening the 
circuit of the supervisory relay, B' . 




FIG. 283.— DETAILS OF AMERICAN ELECTRIC CIRCUIT. 



The talking circuit between two such lines is shown in simplified 
diagrammatic form in Fig. 283, the two sides of the cord being di- 
vided by two two-microfarad condensers in order to prevent the 
signals on the answering side from interfering with those on the 
calling side, and vice versa. 

Referring again to Fig. 280, it will be seen that both supervisory 
lamps will remain dark as long as the subscribers are talking, but as 
soon as either subscriber hangs up his receiver the armature of relay 
S or S', will drop back, thus closing the circuit of the relay, B or B', 
which relay will then receive current over the following circuit : 
From the ungrounded pole of the battery through the retardation 
coil, R', of the line with which the corresponding plug is connected. 
thence through the sleeve contact of the jack and plug to the back 
contact of the relay, S* or S' , and to ground through the coil of the 




350 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 351 

relay, B or B' . Upon the actuation of the relay due to this current 
the circuit of the corresponding supervisory lamp will be complete, 
which lamp will light and remain lighted until the operator pulls 
down the connection. 

The test of this circuit appears to be faulty for the same reason 
as pointed out in connection with the International circuit: That is, 
the test ring is constantly in connection with the line, and therefore 
is subject to all the uncertainties as to its electrical condition as is 
the line itself. The test ring, however, in this case, is maintained 
under normal conditions at a constant potential due to its being 
permanently connected to the underground side of the 40-volt bat- 
tery. When a plug, is inserted into the jack, however, or when a sub- 
scriber's receiver is removed from its hook the potential of this test 
ring is altered, due to the flow of current over the line or over the 
cord circuit and to the consequent drop of potential through the coil, 
R'. When the listening key is thrown the circuit from the tip of the 
calling plug to ground may be traced through the two-microfarad 
condenser on the tip side, through the contact on the tip side of the 
listening key through the operator's receiver and induction coil, 
thence through a 1000-ohm resistance coil to the negative side of the 
battery. The tip of the calling plug will therefore be held by induc- 
tion at practically the same potential as that of the ungrounded side 
of the battery, which is the same potential at which the test rings of 
the free line are supposed to be held. Under these circumstances, 
therefore, no click will follow. If, however, the line is busy, the po- 
tential of its test ring will be lowered, and when the tip of the call- 
ing plug is applied in testing a click will follow. Defective insula- 
tion of the sleeve side of the line with respect to the ground or a par- 
tial cross between the tip and sleeve sides of the line would lower the 
potential of the test ring, and thus cause a false busy test. 

The general appearance of a modern common battery multiple 
switch-board, equipped with lamp signals controlled by line and 
cut-off relays, is shown in Fig. 284, which is taken from a photo- 
graph of the switch-board installed by the Western Electric Com- 
pany for the Bell Telephone Company, of Missouri, in their St. Louis 
exchange. This board consists of 19 sections, with three operators' 
positions at each section. It is finished in mahogany, and is about 
six feet high, four feet wide, with an over-all length of nearly 115 
feet. It is at present wired for 4800 lines, and is capable of accom- 
modating an ultimate number of 5600 lines. 



352 



AMERICAN TELEPHONE PRACTICE. 



In addition to the 4000 multiple calling jacks shown on the 
upper panels of each section there are on the lower panels of 




the switch-board 260 answering jacks and lamps, representing the 
set of subscribers' lines over which the three operators at that sec- 
tion receive their calls. Between the answering jacks and the mul- 



THE COMMON BATTERY MULTIPLE SWITCH-BOARD. 353 

tiple jacks are placed the outgoing trunk jacks, forming the ter- 
minals of trunk lines leading to other offices in the exchange. 

On the horizontal keyboard, below the jacks, is a double row of 
plugs, the rear set or answering plugs being those used for inser- 
tion in the answering jacks in answering a call, and the front set 
being used for testing and afterward connecting with the line of a 
subscriber called for, in the multiple jacks above. The listening and 
ringing keys may be seen directly in front of the plugs. 

The details of the arrangement of apparatus and wiring in cen- 
tral office equipments will be more fully discussed in a succeeding 
chapter. 

In Fig. 285 is shown a front view of somewhat more than one 
section of the great multiple switch-board of the New York Tele- 
phone Company, in its Cortlandt street exchange. In this the 
method of designating the various types of service at the multiple 
jacks is clearly shown. 



CHAPTER XXI. 
TRUNKING SYSTEM BETWEEN COMMON BATTERY OFFICES. 

In the preceding chapter the various methods of connecting two 
subscribers whose lines terminate in a single multiple switch-board 
were discussed. As already pointed out, however, in Chapter XI., 
it is frequently found necessary or expedient to employ more than 
one central office in an exchange, and to make provision for the 
connection of any subscriber in one office with any subscriber in 
any of the other offices. For the purpose of making these connec- 
tions, trunk lines extending between the offices are provided. These 
trunk lines or trunks, as they are more often called, are usually of 
the one-way type — that is, they are used for establishing a connec- 
tion in one direction only. To illustrate: Suppose that there are 
two offices, A and B, in an exchange. Instead of providing a group 
of two-way trunks by means of which all connections between the 
two offices in either direction could be established, two groups of 
one-way trunks are used, one group for calls originating in office 
A, the other for those originating in office B. Each trunk may, 
therefore, be said to have an outgoing and an incoming end, the 
terms "outgoing" and "incoming" always being made with refer- 
ence to the particular office under consideration. Thus exchange 
A will be provided with a number of outgoing trunks and a number 
of incoming trunks. The same will be true of office B. It is evi- 
dent that the outgoing trunk of office A is the incoming trunk of 
office B, and vice versa. 

It has now become the almost universal practice in common bat- 
tery exchanges to terminate the outgoing ends of trunk lines in 
jacks multiplied throughout the subscribers' sections of one office. 
These outgoing trunk lines at one office are the incoming ends at 
the other, and at this latter office the trunk lines terminate in plugs 
and lamps located on the keyboard of the particular position or 
positions. The outgoing ends, therefore, always terminate in jacks 
in front of the operators who look after the subscribers' lines, while 
the incoming ends always terminate in plugs at a special position 
or positions, at which positions multiple jacks of all of the sub- 
scribers' lines in the exchange are placed. 

354. 



TRUNKING SYSTEM BETWEEN COMMON BATTERY OFFICES. 355 

Those operators who sit at the positions of the board at which 
the subscribers' lines terminate in answering jacks and lamps are 
termed, ''subscribers' " operators, or "A" operators, and the posi- 
tions are referred to as "subscribers' " or "A" positions. The posi- 
tions at which the incoming trunk lines terminate are called "trunk" 
positions or "B" positions, and the operators who sit at these posi- 
tions are similarly designated. There is a.- logical reason for ar- 
ranging the outgoing and incoming trunks as described. It is evi- 
dent that in an exchange a subscriber's operator, after answering 
a call, may find that it is for another subscriber's line at that office 
or for a subscriber's line at a distant office, and by the arrangement 
spoken of by which the outgoing lines terminate in multiple jacks, 
she may complete the connection with her calling cord in the same 
manner in either case — that is, by thrusting the calling plug into the 
multiple jack of the local line or trunk line, as the case may be. 
The operators at the incoming end of the trunk lines are special- 
ized in their line of work, and each handles generally from 25 to 30 
incoming trunk lines. On account of the comparatively small num- 
ber of these lines, which must terminate at any incoming trunk oper- 
ator's position, ending them in plugs affords the simplest possible 
means of connecting them with the multiple jacks of the subscribers' 
lines placed before the incoming trunk operators. The incoming 
trunk operators, as a rule, have no means of listening in on a con- 
versation, but each is, however, provided with a telephone set to 
which order-wire lines from other offices are connected. One end 
of each of these order-wire lines terminates in the telephone set of 
an incoming trunk operator at one office, and at the other end is 
multipled through the subscribers' operators' positions at the other 
exchange, terminating at each position in an order-wire key. By 
means of this order-wire key the subscribers' operator may, there- 
fore, connect her telephone set with the telephone set of an incoming 
trunk operator at a distant exchange and thus convey to that oper- 
ator the information as to what connection is to be established over 
the trunk line. 

When a subscriber whose line terminates at one office desires to 
converse with a subscriber whose line terminates in a second office 
he will remove his receiver from its hook in the ordinary way, thus 
lighting his line lamp at a subscribers' operator's position at the 
multiple board of his office. The subscribers', or A operator, will 
answer in the usual way by inserting one of her answering- plugs, 
and learning that the subscriber desired is one whose line terminates 



356 AMERICAN TELEPHONE PRACTICE. 

at a distant exchange, will press the order-wire key of a circuit ter- 
minating in the telephone set of a trunk operator at the desired 
office, and tell her the number of the line with which the connection 
is to be established. Before the A operator releases the order-wire 
key the trunk operator will designate to her over the same order- 
wire the number of the trunk which is to be used in making the 
connection. Thereupon, the A operator at the first office will insert 
the calling plug of the pair used in making the connection, into the 
jack of the trunk line designated, while the trunk operator will test 
the jack of the subscriber's line, and upon finding it not in use, in- 
sert the plug of the same trunk into this jack. The connection is 
now established between the lines of the two subscribers, and it 
remains for the trunk operator to ring the called subscriber, for 
which purpose each trunk line plug is provided with a ringing key. 
A calling supervisory lamp at the subscribers' operator's position in 
the first exchange will light in the same manner as described when 
the calling plug is inserted directly into the multiple of the sub- 
scriber's line. This lamp remains lighted until the called-for sub- 
scriber at the distant exchange responds, but will go out when he 
removes his receiver from the hook, in the same manner as if the 
connection had been made between two lines in the same exchange. 

Associated with each incoming trunk circuit are two lamps — one 
known as the ringing lamp, which lights as soon as the trunk plug 
is inserted into the jack and remains lighted until the called-for sub- 
scriber answers, when it is extinguished, and cannot be relighted 
until the plug is removed from the jack. This l-amp, as its name 
indicates, serves the purpose of showing the trunk operator when 
the called subscriber responds. If this lamp remains lighted for 
a considerable period after she has rung the subscriber she rings 
him again, and continues to do so until he responds or until she is 
convinced that he will not respond. It will be seen that the ring- 
ing lamps at the incoming trunk position and the calling supervisory 
lamp at the subscribers' operator's position at the distant office are 
extinguished simultaneously by the removal of the called sub- 
scriber's receiver from its hook, thus conveying the information to 
both operators simultaneously that the called-for subscriber has 
responded. 

A second lamp associated w T ith each incoming trunk circuit is 
known as the disconnect lamp and lights after a conversation has 
been finished and the connection has been taken down by the sub- 
scriber's operator at the originating office. This lamp also serves 



TRUNKING SYSTEM BETWEEN COMMON BATTERY OFFICES. 357 

in some systems as a guard lamp to prevent mistakes in making 
connections with the trunk lines, as will be pointed out later. 

The method by which the signals for taking down a trunk con- 
nection is given is briefly this: As long as the subscribers are en- 
gaged in conversation both supervisory lamps of the cord circuit 
at the A position remain unlighted as well as the disconnect and 
ringing lamps at the trunk position. The hanging up of the re- 
ceivers of the subscribers operate the supervisory lamps at the A 
position in exactly the same manner as if the lines of the two sub- 
scribers terminated in the same office. In response to the lighting 
of both lamps the operator at the A position will pull down the 
connection. The act of withdrawing the plug from the outgoing 
trunk jack will cause the lighting of the disconnect lamp corre- 
sponding to that trunk line at the distant B position. When this 
disconnect lamp lights the trunk operator withdraws the plug from 
the line of the called subscriber, thus restoring all apparatus to its 
normal condition. 

As no act on the part of either subscriber will have any effect on 
the disconnect lamp at the incoming trunk position, it will be seen 
that the A operator has complete charge of the connection after the 
called subscriDer has answered, the B operator receiving the signal 
for disconnection when the A operator takes down the connection 
at her position. 

When the trunk operator receives the order from the subscriber's 
operator at a distant exchange for connection with a certain line 
she tests the jack of that line with the trunk plug in the same man- 
ner as the subscriber's operator tests with the calling plug when a 
connection is to be completed at her own board. If the trunk oper- 
ator finds the line free she completes the connection in the manner 
already described. If, however, she finds it busy she will insert the 
trunk plug into a "busy back" jack, one or more of which are located 
within the reach of each trunk operator. This jack is constantly 
connected with a source of interrupted e.m.f., which will place a 
"tone" on the line. The placing of the trunk plug in the "busy 
back" jack will also cause the calling supervisory lamp at the sub- 
scriber's position at the distant office to flash at regular intervals. 
The calling subscriber, hearing the tone, will, if he understands its 
significance, hang up his receiver, thus lighting the supervisory lamp 
associated with the answering plug used in making the connection, 
whereupon the operator, seeing both supervisory lamps lighted, will 
pull down the connection. If the subscriber does not understand 



358 



AMERICAN TELEPHONE PRACTICE. 



the significance of the tone, the A operator, seeing the periodic flash- 
ing of the calling supervisory lamp, will throw her listening key and 
inform the subscriber that the line is busy. 

Some systems also employ what is termed a "don't answer" jack, 
which works in a manner similar to that described for the "busy 
back'' jack. The "don't answer" jack is used by the trunk operator 
if the subscriber whom she has been calling does not respond. By 
placing the trunk plug into this jack a tone different from the busy 
back tone is put on the line, and the calling supervisory lamp at the 
distant station is caused to flash at either a slower or more rapid 
rate, so that both the operator and the calling subscriber may under- 
stand the meaning of the signals. 



TRUNK LINE 



NCOMIN5 TRUNK LINE. 




FIG. 2S6.— WESTERN ELECTRIC TRUNK CIRCUIT. 

The standard trunk circuit of the Western Electric Company, 
used in connection with the subscriber's line and cord circuits, 
shown in Figs. 264, 265 and 266, is shown in Fig. 286. In this the 
outgoing end is shown at the left of the figure, the apparatus con- 
sisting merely of jacks multipled throughout the regular subscrib- 
ers' positions at one office. The apparatus at the incoming end of 
the trunk line is shown at the right of the figure, and, as will be 
seen, the entire complexity of the circuit is thus placed at the in- 
coming end. 

It will be seen that at the incoming end the two sides of the trunk 
circuit are separated by a repeating coil of the ordinary common 
battery type. Between the windings connected directly across the 
trunk line is bridged a two-microfarad condenser, and in multiple 



TRUNKING SYSTEM BETWEEN COMMON BATTERY OFFICES. 359 

with this, one winding of a relay, A, this winding having a resist- 
ance of about 12,000 ohms. Between the two windings of the re- 
peating coil, which are connected with the incoming trunk plug, 
is bridged the common storage battery of the office. Included be- 
tween the ring contact of the trunk plug and one terminal of the 
repeating coil is a supervisory relay, B, this relay having, it will be 
seen, the same circuit connection, so far as these windings are con- 
cerned, as the regular supervisory relays in the subscribers' oper- 
ator's cord circuits at the A position. The remaining features of 
the incoming trunk circuit can be best described in studying their 
operation. 

Fig. 287 shows the complete circuit of two subscribers' lines ex- 
tending to different offices, connection being made between the two 
by the subscribers' operator's cord circuit at the A position of one 
exchange, and the trunk circuit extending between the two ex- 
changes. If, after answering a call, the A operator ascertains that 
the connection is for a subscriber in a distant exchange, she will by 
order wire inform the operator at that exchange and receive in 
return the number of the trunk to be used. She will, therefore, at 
once insert the calling plug into the outgoing jack of the trunk desig- 
nated, and the trunk operator will insert the trunk plug into the mul- 
tiple jack of the subscriber's line. Assuming, that the originating or 
A operator inserted the plug into the outgoing trunk jack before the 
trunk operator inserted the trunk plug in the subscriber's line, the 
following conditions would exist : The relay, A, would be energized, 
since it receives current from the battery at the originating office over 
the metallic circuit of the trunk line. This relay will attract its arma- 
ture and place a 40-ohm shunt about the disconnecting lamp to 
prevent its subsequent illumination. It is evident, therefore, that no 
matter whether the circuit of the disconnect lamp is completed or 
not, it will not be illuminated until the relay, A, is de-energized, so 
as to remove the shunt from the lamp. It is furthermore evident that 
the relay, A, cannot be de-energized as long as the calling plug is in 
the outgoing jack at the originating office. The calling supervisory 
relay, R, of the A operator's cord circuit at the first exchange will not 
be operated by the current flowing through it over the metallic circuit 
of the trunk line on account of the very high ressitance of the relay, 
A, at the incoming trunk position. As soon as the B operator in- 
serts the trunk plug into the line of the called subscriber a circuit 
will be closed from the ungrounded side of the battery at the second 
office through the ringing lamp, and through the winding of the re- 




-AWV— -p 



U ko" izooo- 







FIG. 2S7.— CONNECTION BETWEEN TWO SUBSCRIBERS THROUGH 
TWO OFFICES— WESTERN ELECTRIC SYSTEM. 



TRUNKING SYSTEM BETWEEN COMMON BATTERY OFFICES. 361 

lay, C, to the third contact on the plug to the test ring of the multiple 
jack of the subscriber's line, thence through the cut-off relay of the 
subscriber's relay to ground. The closure of this circuit will serve 
three functions: It will illuminate the ringing lamp; it will operate 
the relay, C, and it will operate the cut-off relay. The illumination 
of the ringing lamp shows the trunk operator that the subscriber has 
not yet responded. The operation of the relay, C, will close •the 
normally open tip strand of the trunk cord cricuit, and cut off the 
normally closed circuit from the tip of the trunk plug through the 
operator's induction coil to ground. 

The operation of this relay, C, will also close a circuit which may 
be traced from ground through one of the, movable levers of this 
relay through the disconnect lamp and the resistance -coil, r, to the 
negative pole of the battery. This will not, however, cause the 
illumination of the disconnect lamp, because the lamp is shunted by 
resistance, r, which is brought into circuit by the operation of the 
high resistance relay, A. The operation of the cut-off relay will per- 
form the ordinary function with respect to the subscriber's line of 
cutting off all signaling apparatus normally connected with it. The 
conditions, as they exist, may be restated as follows : The disconnect 
lamp is not lighted because shunted by resistance, r. The ringing 
lamp is lighted, and the test relay, C, is operated so as to keep the 
talking circuit closed through the tip strand of the trunk. The test 
ring of the multiple jacks of the subscriber's line is raised to a po- 
tential above that of the earth, by virtue of being connected with 
the ungrounded side of the common battery through the third strand 
of the cord, and therefore the subscriber's line will test busy at all 
sections of the board. The trunk operator, after ringing the called 
subscriber in the ordinary manner, will be informed of his response 
by the going out of the ringing lamp, which will occur when the sub- 
scriber removes his receiver from its hook for the following reasons : 
Current will flow from the battery over the circuit of the line and thus 
pass through the coil of the relay, B, located in the sleeve strand of 
the trunk circuit. This relay will thus be under the control of the 
subscriber, and will remain actuated as long as he keeps his receiver 
removed from its hook. 

The attraction of the armature of this relay will close a circuit, in- 
cluding the 6o-ohm coil of the relay, D, from the ground side of bat- 
tery at the first office over the tip side of the trunk line through the 
contact of the relay, B, through the 6o-ohm winding of the relay. D, 
the low resistance winding of the relay, A, to the sleeve side of the 



362 AMERICAN TELEPHONE PRACTICE, 

trunk line, and to the ungrounded side of the battery at the first office. 
The operation of the relay, D, will close the circuit through a 40-ohm 
winding on this same relay, which winding will thus be placed in 
shunt with the ringing lamp, which will cause that lamp to go out. 
The relay, D, is therefore self-locking, as it holds the circuit through 
its own winding closed as long as operated. This relay will there- 
fore remain operated until the end of the connection, regardless of 
whether relay, B, is operated or not. It is evident that the energiza- 
tion of the relay, A, will be maintained as long as the calling plug at 
the distant exchange remains in the trunk jack. The relay, A, there- 
fore, cannot be released until the called subscriber has hung up his 
receiver and the subscriber's operator has withdrawn her plug from 
the trunk jack. During conversation both trunk lamps are out. The 
calling supervisory lamp at the A operator's position at the first 
office was also put out by the action of the called subscriber in 
removing his receiver from its hook. This action, it will be re- 
membered, energized the relay, B, which, in closing its contact, 
completed the low resistance shunt abount the 12,000-ohm winding 
of the relay, A, this shunt including the 60-ohm winding of the 
relay, D, and the 20-ohm winding of the relay, A. Enough current 
was therefore allowed to flow through the supervisory relay, R, at 
the first exchange to attract its armature, thereby shunting the call- 
ing supervisory lamp and causing it to go out. 

In case the B operator in the second exchange completed the con- 
nection between the trunk line and the subscriber's line before the 
A operator at the first exchange inserted the calling plug into the 
designated trunk line, the disconnect lamp in the trunk circuit would 
be lighted to be immediately put out when the connection was 
completed by the A operator. This serves to some extent as a 
guard against mistakes in making connection with the trunk lines. 
It sometimes happens that the A operator will mistake the number 
of the trunk designated by the B operator and plug into the wrong 
trunk. She would probably notice the error if she attempted to 
plug into a busy trunk, because of the test. If, however, the trunk 
into which she plugged by mistake was not busy, she would not 
notice the error. In that case the B operator would notice that the 
disconnect lamp, as well as the ringing lamp in the trunk, the plug 
of which she has inserted into the multiple jack, was lighted, and the 
A operator, seeing from the fact that the calling supervisory lamp 
did not go out, that the subscriber did not answer, would again 
speak to the B operator over the order wire. 



TRUNKING SYSTEM BETWEEN COMMON BATTERY OFFICES. 363 

At the end of the conversation the hanging up of the receiver of 
the called subscriber will light the calling supervisory lamp at the A 
operator's position at the office in the usual manner. The hanging 
up of the called subscriber's receiver will cause the de-energization 
of the relay, B, in the incoming trunk circuit at the second office and 
falling, back of its armature will open the low resistance circuit 
about the 12,000-ohm winding of the relay A, and cause relay R at 
the first office to release its armature. This will light the calling 
supervisory lamp. The opening of the contact of relay B at the 
trunk position does not cause the de-energization of the relays A or 
D, because battery will continue to flow through the high resistance 
winding of the former, and relay D will be locked by current flow- 
ing through its contact and 40-ohm winding, the winding of relay 
C and the cut-off relay. Both trunk lamps will therefore remain 
dark. As soon, however, as the A operator withdraws the calling 
plug from the jack of the trunk line the relay A will release its arma- 
ture, which will remove the shunt, r', from about the disconnect 
lamp and allow the illumination of this lamp over a circuit which 
may be traced from the ungrounded side of the battery through the 
resistance coil, r, the lamp itself, to ground through one contact of 
the relay C. Seeing the disconnect lamp light the trunk operator 
will remove the trunk plug from the jack, thus allowing the cut-off re- 
lay of the line to assume its normal position, and rendering the line 
ready to receive a call. The breaking of the circuit between the 
third strands of the plug and the test ring of the multiple jack will 
de-energize the relay, C, which will release both of its contact levers, 
thus restoring all circuits to their normal condition. 

When a trunk plug is not in use the relay, C, being de-energized, 
the circuit from the tip of the trunk plug is not connected with the 
repeating coil, as is usual, but is connected to ground through one of 
the back contacts of the relay, C, and the third winding of the B 
operator's induction coil. This is for the purpose of allowing the B 
operator to test the multiple jack of the subscriber's line to ascertain 
whether or not it is busy, the test, of course, depending on whether 
the test rings of the line tested are raised to a potential above that Ci 
the earth, as would be the case when a plug was connected to one of 
the jacks at another section, or at the same potential as the earth, as 
would be the case were the line idle. As soon as the operator plugs 
into the jack, however, the operation of the relay, C, cuts off all 
connection with the talking circuit and establishes the proper con- 
nection for talking over the trunk circuit by connecting the tip side 



364 AMERICAN TELEPHONE PRACTICE. 

of the trunk from the plug to one winding of the repeating coil. The 
B operator's telephone set is available only for receiving and trans- 
mitting order-wire communications over one of the order-wire cir- 
cuits. One of these order-wire circuits is shown extending from 
the order-wire key, 0, at the A operator's position to the B oper- 
ator's telephone receiver and secondary coil at the trunk position. 
The closure of the key, 0, will therefore permit the two operators 
to communicate for the purpose of allowing the A operator to 
designate the subscriber's line called for to the B operator, and the 
B operator to designate to the A operator the number of the trunk 
to be used. 

It is customary to arrange at each incoming trunk position some 
means of transferring the incoming order-wire calls in case the trunk 
operator leaves her position either temporarily or for a period of 
considerable length. The scheme which is used by the Western 
Electric Company for this purpose is as follows : If the subscriber's 
operator at one office presses the order-wire key, and upon attempt- 
ing to converse with the trunk operator at the second office receives 
no response, she will press the key, K, called a "ringing-down" key, 
at the same time that she depresses the order-wire key, 0. which will 
connect the generator momentarily with the order wire. This will 
send current from the generator over the order-wire circuit and throw 
an electrical restoring drop, E, bridged across the circuit of the order- 
wire line. This drop is used simply as a relay to light the pilot lamp 
at the trunk position, which lamp will remain lighted until the opera- 
tor, seeing it, responds. The lamp is extinguished by depressing 
the key, K lf which closes a circuit through the restoring winding 
of the drop, thus opening the lamp circuit. This order-wire pilot 
lamp is connected to the night bell circuit in a similar manner to 
that in which the line pilot lamps are wired, so that the night alarm 
will ring whenever the order-wire lamp lights, if the night alarm 
switch is in the proper position. 

In the operation so far described the ringing of the called sub- 
scriber has always been done manually by the A operator when no 
trunk was used in making a connection, or by the B operator where 
connection was made through a trunk. In some of the exchanges 
built by the Western Electric Company this necessary act of ringing 
on the part of the B operator is partially or entirely eliminated, and 
the method by which this is accomplished is called "automatic" ring- 
ing. An automatic ringing device, as applied to an incoming trunk 
cord circuit, is shown in Fig. 288, from which figure all portions of 



TRUNKING SYSTEM BETWEEN COMMON BATTERY OFFICES. 365 



the apparatus save that actually concerned in ringing the called sub- 
scriber are omitted. 

In this the ringing key, K, is provided with a clutch magnet, m, 
which, when the key is depressed into its ringing position, after the 
plug has been inserted in a jack, holds the springs of the key in this 
position and allows ringing current to flow from one brush of the 
generator, G, through the interrupter, /, the winding of relay, R, 
over the ring side of the line, through the bell and condenser at the 




j<m* t 



FIG. 288.— WESTERN ELECTRIC AUTOMATIC RINGING CIRCUIT. 



subscriber's station and back on the tip side of the line to the 
grounded generator brush. The interrupter is constantly revolving. 
It is arranged so that the circuit of the generator will be closed to 
the ringing key for one second out of every six, but the resistance of 
the bell and condenser in the subscriber's instrument is great enough 
to prevent the ringing current from operating relay, R, and thus re- 
storing the key. The interrupter is so arranged that during the 
other five seconds of each revolution the live or non-grounded side 
of the battery is connected to the wire leading to the ringing key. 



366 



AMERICAN TELEPHONE PRACTICE. 



When, therefore, the ringing key is depressed the subscriber's bell 
rings at intervals of six seconds for a period of one second, the bat- 
tery being thrown on the line during the five-second intervals. Bat- 
tery current, however, finds no path, on account of the presence of 
the condenser. As soon, however, as the subscriber responds by 
removing his receiver from its hook the current from the battery, or 
from the ringing generator, whichever happens to be flowing at the 
time, passes through the magnet of the relay, R, and trips the ring- 
ing key, thus cutting off ringing current before the subscriber has 
time to get his receiver to his ear. By this means the bell of the 



TRUNK. 



ToTtsT W£\.*.Y 




FIG. 289.-KELLOGG TRUNK CIRCUIT. 



subscriber is rung at short intervals' Until he responds without any 
further attention on the part of the operator. 

While the system just described is termed "automatic" ringing, 
this term is not strictly appropriate, because the ringing key is oper- 
ated manually by the operator, the only automatic feature being the 
intermittent ringing and the disconnection of the ringing generator 
from the line. It is, however, quite easy to arrange for strictly auto- 
matic ringing, and this will be described in connection with a later 
system, although in some cases it has been and is applied to West- 
ern Electric circuits. 

In Fig. 289 is shown the trunk line circuit employed by the Kel- 
logg Switchboard and Supply Company, as used with their two- 
wire common battery multiple circuits, already described. 



TRUNKING SYSTEM BETWEEN COMMON BATTERY OFFICES. 367 

In this the trunk line consists of two wires, forming a metallic 
circuit, as in the Western Electric system. At the outgoing end 
the trunk terminates in two-point jacks, multipled throughout the 
sections in the ordinary manner, the sleeve contact, however, of all 
the jacks belonging to a line grounded through an impedance coil, 
this being for the purpose of securing the operation of the relay, R*, 
shown in Fig. 270, in Chapter XX. 

The trunk circuit is divided by means of a repeating coil, that por- 
tion of the coil which is connected with the trunk line proper having 
connected between its two halves a high resistance relay, A, which 
is operated as soon as the calling plug of the subscriber's cord cir- 
cuit at the distant exchange is inserted into an outgoing trunk jack. 
It would be impossible to talk through the winding of this relay, 
but arrangements are made for short-circuiting it by the contacts 
of another relay, which condition is maintained throughout the time 
of the conversation. The other side of the repeating coil, which is 
connected with the trunk plug, has connected between its windings 
a two-microfarad condenser. The operation of this circuit will be 
best understood from the consideration of Fig. 290, which shows 
the circuit of two subscribers' lines connected through an A oper- 
ator's cord circuit at the first exchange, and through a trunk line 
terminating in a plug at a second exchange. For full particulars 
of these line and cord circuits reference is made to the description 
accompanying Figs. 268, 269 and 270 in Chapter XX. 

Referring, therefore, to Fig. 290, when the A operator at the first 
office receives an order from a subscriber whose line is shown, for 
a connection with a subscriber at the second office, after she has 
designated to the incoming trunk operator at the distant office the 
number of the line and received from that operator the number of 
the trunk to be used, the A operator will insert the calling plug of 
her pair of cords into the outgoing trunk jack designated. Assum- 
ing that the A operator inserts the calling plug into the outgoing- 
jack before the B operator inserts the trunk plug into the subscrib- 
er's multiple jack, the following conditions exist: The relay, A, at 
the incoming trunk end will be operated from current received from 
battery, B', at the first exchange, over the metallic circuit of the 
trunk line, causing this relay to attract its armature. This will cause 
the lighting of the disconnect lamp over a path which may be traced 
from ground through the back contact of the relay, C, the front 
contact of the relay, A, and the disconnect lamp to the negative 
side of the battery, B 2 , at the second office. On account of the 



m\ 




FIG. 290.— CONNECTION BETWEEN TWO SUBSCRIBERS THROUGH TWO 
OFFICES— KELLOGG SYSTEM. 



TRUNKING SYSTEM BETWEEN COMMON BATTERY OFFICES. 369 

very high resistance of the relay, A, the current flowing over the 
metallic circuit of the trunk line will not operate the supervisory 
relay, R 3 , of the A operator's cord circuit which will, therefore, 
leave its contact in the calling supervisory lamp circuit closed. The 
other supervisory relay, R 4 , of the A operator's cord circuit will, 
however, be operated, current flowing from the live side of the 
battery, B', through the winding of this relay in multiple through 
the winding of relays, A and R 3 , and the retardation coil connected 
between the sleeve side of the outgoing trunk jacks and ground. 
The calling supervisory lamp, S' 2 ,o( the A operator's cord, will, there- 
fore, be lighted. When the B operator inserts the trunk plug into 
the multiple jack of the called subscriber the relay, C, will be at 
once operated over a circuit which may be traced from ground at 
the battery, B 2 , through the winding of the relay, C, over the sleeve 
side of the cord circuit to the sleeve of the multiple jack, thence to 
ground through the cut-off relay of the subscriber's line. The 
operation of this relay will, by attraction of its right-hand contact 
lever, cause the breaking of the circuit of the disconnect lamp, 
therefore causing that lamp to go out. The right-hand contact 
lever of the relay, C, will, when thus attracted, close a contact which 
will complete a circuit which may be traced from ground through 
the front contact of the right-hand lever of this relay through the 
back contact of the relay, D, not yet energized, thence through the 
ringing lamp to the live side of the battery. This will cause the 
illumination of the ringing lamp. When the subscriber called for 
responds the relay, E, will at once attract its armature on account of 
current flowing in the metallic circuit of the line. This relay is, 
therefore, under the control of the subscriber. By attraction of 
its left-hand lever the winding of the relay, A, will be short-circuited, 
which relay will thus allow its armature to drop back. This will 
not, however, affect the disconnect lamp circuit, which will be held 
open at the right-hand back contact of the relay, E. The short- 
circuiting of the winding of relay, A, will cause the operation of 
the relay, R 3 , in the subscribers' operator's cord circuit, and will, 
therefore, extinguish the calling supervisory lamp at the A position 
at the distant exchange. The closing of the contact by the right- 
hand lever of the relay, E, will cause current to pass through the 
relay, D, which will open the circuit of the ringing lamp, showing 
the trunk operator that the subscriber has responded. By the at- 
traction of the left-hand lever of the relay, C, the normal test cir- 

24 



•370 AMERICAN TELEFHONE PRACTICE. 

cuit will be broken, and the talking circuit between the tip of the 
trunk plug and the repeating coil will be established. 

It was shown that when the A operator plugged into the trunk 
jack before the B operator plugged into the subscriber's line the 
disconnect lamp would momentarily light, to be extinguished when 
the B operator completed the connection. It will also be seen that 
if the B operator completes the connection first the disconnect lamp 
will be again momentarily lighted, to be again put out when the 
A operator completes the connection with the trunk. This is true, 
because the relay, C, will be operated by the act, on the part of the 
trunk operator, of inserting the trunk plug into the multiple jack, 
while the relay, A, will be inert as will the relay, E. The circuit 
through the disconnect lamp will, therefore, be completed from 
the back c mtact of the relay, C. As soon, however, as the A oper- 
ator plugs into the trunk the relay, A, will be operated, which will 
put out the disconnect lamp. It will be seen, therefore, that this 
is an additional guard against a mistake in the connection of the 
trunk line, because if the B operator notices that the disconnect 
lamp lights and remains lighted at the beginning of the connection 
she will obey its signal and pull down the trunk plug. In this case 
the calling subscriber will remain without his connection, where- 
upon the A operator will again communicate with the B operator 
for a re-designation of the trunk. 

During conversation the following conditions exist: Both super- 
visory lamps at the A position of the first office are extinguished. 
The relay, A, at the incoming trunk position of the second office 
is not energized because short-circuited. The relay, C, is operated 
and will remain operated as long as the trunk plug is inserted in 
the line jack. The relay, E, is operated and is under the control 
of the called subscriber, as it will be released as soon as he hangs 
up his receiver. The relay, D, which controls the circuit of the ring- 
ing lamp is operated and will remain so as long as the connection 
exists, because when operated it closes a circuit from battery 
through its own winding, its front contact to ground through the 
now closed contact of the relay, C. It is evident, therefore, that 
the ringing lamp will not light as long as the connection exists, 
and cannot be again lighted until the relay, C, is de-energized, 
which will only take place when the trunk plug is removed from the 
jack at the end of conversation. 

When the called subscriber hangs up his receiver the relay, E, 
will be de-energized, thus removing the short circuit from about 



TRUNKING SYSTEM BETWEEN COMMON BATTERY OFFICES. 371 

the relay, A, and causing the attraction of its armature. The dis- 
connect lamp will not be lighted, however, because "of the ener- 
gization of the relay, C, which keeps the ground circuit leading to 
the disconnect lamp open. The breaking of the short circuit about 
the relay, A, will, however, cause the relay, R 3 , to release its arma- 
ture, thus causing the lighting of the calling supervisory lamp of 
the A operator. The calling supervisory lamp at the first office is, 
therefore, under the control of the called-for subscriber whose line 
terminates in the second office. When both the supervisory lamps 
of the A operator are lighted the operator knows that both sub- 
scribers have finished the conversation and withdraws the plugs. 
The removal of the calling plug from the outgoing trunk jack de- 
energizes the relay, A, which allows its armature to fall back and 
thus light the disconnect lamp. The path over which this lamp 
is illuminated includes the front contact of the right-hand armature 
of the relay, C. The illumination of the disconnect lamp is a signal 
for the operator to pull down the connection, and in doing so the 
relay, C, is de-energized, thus putting out the disconnect lamp and 
leaving all apparatus in its normal condition. 

It will be seen that the relay, C, when not operated leaves the tip 
of the trunk plug connected through the test relay to ground, thus 
securing a test similar to that in connection with the calling plug 
of the A operator already described. 

In offices of the Kellogg system, in which there are two or more 
trunk positions, the telephone set of each trunk operator is con- 
nected to a two-way key. If this key at any position is thrown in 
one direction and an operator at a distant office presses an order- 
wire key and attempts to converse with that operator, the order 
will be received by the operator, and at the same time an order-wire 
pilot lamp will be lighted at this trunk operator's position, this 
order-wire pilot lamp being operated by a relay energized by the 
closure of the order-wire key at the distant station. However, if 
the key is thrown in the opposite direction, the order and pilot lamp 
signal will be transferred to another trunk position. If the key at 
this second position is thrown in one direction the order and pilot 
lamp signal will be received in this second position, or, if thrown 
in the other direction, it will be transferred to the third position. 
In this way if the trunk operator leaves her position either mo- 
mentarily or permanently she may, by throwing this transfer key, 
cause both the order-wire signals and the orders coming over her 
order-wires to be transferred to some other operator. In this way 



372 



AMERICAN TELEPHONE PRACTICE. 



it is possible for one trunk operator to do all of the incoming trunk 
business at night or whenever the number of incoming trunk calls 
is small. 

As in the Western Electric system, the order-wire pilot lamp is 
so wired,' in connection with the night alarm, as to ring whenever 
the order-wire pilot lamp lights, if the night alarm switch is in the 
proper position. 

Automatic ringing, as applied to some of the European work in- 
stalled by the Kellogg Company, is so arranged that the generator 
will be thrown into circuit with the subscriber's line by the act of 
inserting the trunk plug into the jack of the called subscriber, the 
operator not being required to press a ringing key. This method 




FIG. 291.— KELLOGG AUTOMATIC RINGING CIRCUITS. 

of operating is brought about by means of the circuit shown in 
Fig. 291. 

In this it will be seen that the ringing current is thrown on to 
each cord circuit by an extra armature of the relay, C, which is in 
the sleeve side of the cord, which relay is always actuated when 
the operator plugs into the jack. A second relay, F, placed in the 
circuit of the ringing current itself, serves, when actuated by the 
subscriber's removal of his receiver from its hook, to cut off the 
ringing current in a manner similar to that described in connection 
with the Western Electric automatic ringing device, Fig. 288. 

In Fig. 292 is shown the trunk circuit used in some of the recent 
exchanges installed by the Stromberg-Carlson Company. This, it 
will be seen, is quite different from the trunk circuits already de- 
scribed, it having a considerable amount of apparatus at the out- 
going end, this end including, beside the multiple jacks, a repeating 



TRUNKING SYSTEM BETWEEN COMMON BATTERY OFFICES. 373 



coil, condenser and two relays. The incoming end also comprises 
a repeating coil, condenser and the usual apparatus for actuating 
the disconnect and ringing lamps, as well as the operator's telephone 
set, ringing keys, etc. 

In Fig. 293 two subscribers' lines terminating at different offices 
are shown through a trunk circuit extending between these offices. 

Remembering that the relay, S z , is energized when the answering 
plug of the A operator's cord is inserted in the jack of the calling 
subscriber's line, it will be seen that if the A operator inserts the 
calling plug into the outgoing trunk jack before the B operator in- 
serts the incoming trunk plug into the jack of the subscriber's line 



ar^ar^' ffi 




OUT-GOING TRUNK 



FIG. 



-STROMBERG-CARLSON TRUNK CIRCUIT. 



called for, the following conditions will exist: The relay, A, will 
be operated by current flowing from the battery through the wind- 
ing of the relay, S 3 , to ground, through the winding of relay, A. 
The operation of the relay, A, will cause the flow of current from 
the ungrounded side of battery through the coil of relay, B, to the 
center point of the outgoing trunk repeating coil, thence over both 
sides of the trunk line in multiple to the center point of the incom- 
ing trunk repeating coil, thence to ground through the relay, C, at 
the incoming trunk end. The relay, C, will thus be operated, but 
the relay, B y will not be operated because of the very high resistance 
of the winding of the relay, C, through which the relay, B, is not 
capable of acting. It will thus be seen that the relay. A, at the 



4V#r 




l#K 



FIG. 293.— CONNECTION BETWEEN TWO SUBSCRIBERS THROUGH 
TWO OFFICES— STROMBERG-CARLSON SYSTEM. 



374 



TRUNKING SYSTEM BETWEEN COMMON BATTERY OFFICES. 375 

outgoing end, and the relay, C, at the incoming end will be ener- 
gized when the A operator plugs into the outgoing jack, and will 
remain energized as long as the connection remains at the outgoing 
end. The operation of the relay, C, closes the circuit of the dis- 
connect lamp, this circuit being traced from the negative side of 
battery at the second office through the disconnect lamp, the 
left-hand back contact of the relay, D, to ground through the con- 
tact of relay, C. The energization of relay, C, also causes the il- 
lumination of the ringing lamp, current flowing from the negative 
side of battery through this lamp, the back contact of relay, E, the 
closed right-hand contact of relay, D, to the grounded side of bat- 
tery through the closed contact of the relay, C. 

The calling supervisory lamp in the A operator's cord circuit 
was illuminated by the plugging in of the A operator, because 
the supervisory relay, S 2 , was not energized on account of the 
presence of the condenser, K, between the windings of the re- 
peating coil in the outgoing end of the trunk. The condi- 
tions will be as follows, therefore, when the connection is 
made at the outgoing end, but not yet made at the incoming; 
end. The calling supervisory lamp in the A operator's cord 
will be lighted, as will also the disconnect and ringing lamps at the 
incoming end of the trunk. As soon as the B operator plugs into 
the multiple jack of the called subscriber the relay, D, will be oper- 
ated over a circuit which may be traced from the ungrounded side 
of the battery through the coil of this relay over the tip contacts 
of the plug and jack to ground through one-half of the cut-off relay 
winding. The operation of the relay, D, will open the circuit of 
the disconnect lamp, thus extinguishing it, the ringing lamp, how- 
ever, will remain lighted, but the circuit through this lamp to the 
grounded side of battery will now be established through a con- 
tact of relay, D, instead of a contact of relay, C. After ringing the 
called subscriber the operator watches the ringing lamp, which will 
remain lighted until that subscriber responds. The act of raising 
his receiver from its hook will cause the energization of the relay, F, 
as the talking current for the called subscriber's line will pass 
from the live side of the battery through the coil of the relay, F, 
over the sleeve side of the line, thence back over the tip side of the 
line to ground through the coil of the cut-off relay in the line cir- 
cuit. The operation of the relay, F, will cause the energization of 
the relay, E, which will break the circuit of the ringing lamp, causing 
it to go out. As soon as the relay, E, is operated it completes, by 



376 AMERICAN TELEPHONE PRACTICE. 

its own contact, a circuit through its coil to ground, which circuit 
also includes the front contact of the relay, D. Thus relay, £, when 
once operated, cannot be de-energized until the relay, D, is de- 
energized, which means that the ringing lamp after having been put 
out cannot be again lighted until the trunk plug is withdrawn from 
the jack. 

It will be remembered that when the calling subscriber responded 
the relay, F, was operated, which completed the circuit from the. 
retardation coil, I 1 , to ground, this path being in multiple with the 
high resistance coil of the relay, C, between the middle point of the 
incoming trunk repeating coil and ground. A low resistance path 
through the retardation coil, F, is therefore furnished for the current 
through the relay, B, which relay is now for the first time operated 
and serves to short-circuit the condenser, K, bridged across the out- 
going trunk repeating coil. This furnishes a low resistance path 
across the outgoing end of the trunk which causes the operation 
of relay, S 2 , thus extinguishing the calling supervisory lamp at the 
office at which the call originated. It will thus be seen that the 
notification of the response of the called subscriber is given to the 
incoming trunk operator by the going out of the ringing lamp, and 
to the A operator at the distant office by the going out of the calling 
supervisory lamp. 

The condition then during conversation is that both the answer- 
ing and the calling supervisory lamps of the A operator's cord cir- 
cuits, and the disconnect and the ringing lamps at the incoming 
end of the trunk line are out. At the end of the conversation the 
hanging up of the calling subscriber's receiver will, of course, illu- 
minate the answering supervisory lamp of the A operator's' cord 
circuit. The hanging up of the called subscriber's receiver will pro- 
duce no effect in so far as the signals are concerned at the incoming 
trunk end, but will cause the lighting of the calling supervisory lamp 
in the A operator's cord circuit at the distant office in the follow- 
ing manner: When the called subscriber hangs up his receiver 
the relay, F, will be de-energized, thus breaking the circuit to 
ground through the impedance coil, /', and causing the relay, B, at 
the outgoing end of the trunk line to release its armature, which, 
by breaking the shunt about the condenser causes the supervisory 
relay, S 2 , of the A operator's cord circuit to release its armature, 
thus lighting the calling supervisory lamp. 

In response to the lighting of both supervisory signals the A 
operator will pull down the connection which will cause the illu- 



TRUNKING SYSTEM BETWEEN COMMON BATTERY OFFICES. 377 

mination of the disconnect lamp at the incoming end of the trunk 
line, for the following reasons: The breaking of the connection 
between the outgoing trunk jack and the calling plug will cause the 
de-energization of the relay, A, which, by releasing its armature, 
will break the circuit through the relay, C, at the distant office, 
causing it to release its armature and thus completing the circuit 
of the disconnect lamp from ground, through the front contact of 
the left-hand lever of the relay, D. In response to this signal the 
incoming trunk operator will pull down the connection, thus de- 
energizing the relay, D, which, by releasing its armature, will put 
out the disconnect lamp and will also open the circuit of the lock- 
ing relay, E, thus restoring all apparatus to its normal condition. 

It has been pointed out that if an A operator plugged into the 
outgoing end of the trunk before the B operator plugged into the 
subscriber's multiple jack, the disconnect and ringing lamps at the 
incoming end would light, but the disconnect lamp would imme- 
diately, go out when the B operator plugged into the called sub- 
scriber's jack. It will also be seen that if the B operator completes 
the connection at her end before the A operator plugs into the 
trunk the disconnect and ringing lamps will also light, owing to 
the energization of the relay, D. In either case, therefore, both 
lamps will be lighted whenever either operator completes the con- 
nection before the other, and the disconnect lamp will go out when 
the connection is completed at both ends, thus securing the advan- 
tages of guarding against mistakes in a manner similar to that of 
the Kellogg system. This circuit operates in accordance with the 
standard code of signaling employed in modern work, but it has 
the disadvantage of requiring two repeating coils through which 
transmission must always be effected. This is undesirable, though 
not fatal. 



CHAPTER XXII. 

THE DIVIDED MULTIPLE SYSTEM. 

It has been shown in preceding chapters that the capacity of the 
types of multiple board so far considered is limited in practice by 
the size of the spring jack used. Where ^-inch jacks are used, 6000 
lines has been usually considered the ultimate capacity of the switch- 
board. Similarly, switch-boards with a capacity of 9000 lines have 
been constructed using f-inch jacks, and with a capacity of 18,000 
lines using j-^-inch jacks. Where occasion demands it is possi- 
ble to reduce the size of the jack to J inch, making possible a switch- 
board capacity of 24,000 lines. Beyond this it does not at present 
seem desirable to go in the reduction of the size of the jack on ac- 
count of difficulties, from both constructive and operative stand- 
points. 

A method of increasing the possible size of multiple switch- 
boards has been devised by Mr. Milo G. Kellogg, and is now work- 
ing in two large exchanges in this country. In this system, which 
is known as the ''divided multiple system,'' the switch-board and 
lines are divided into a number of groups, usually four. In the 
four-division divided multiple board the switch-board consists of 
four multiple boards, all in the same building, and to each of these 
is connected one of the groups of lines in the same manner as if 
they were in separate offices. Primarily, therefore, we have in 
this arrangement four groups of lines, each group terminating in 
a multiple switch-board of sufficient size to properly handle all of 
the lines in that group. With this arrangement, if no further pro- 
vision were made, it would be possible for any subscriber in one 
division to obtain a connection with another subscriber in his own 
division, but not with any subscriber in any of the three other 
divisions. In order to enable any subscriber in, say, the A division, 
to obtain a connection with a subscriber in the B, C or D divison, 
means are provided, not for trunking between the divisions, but 
for enabling the calling subscriber to signal directly to one of the 
operators in that division in which the line of the desired subscriber 
belongs. Each line is therefore provided with a signal in each di- 
vision of the board, and the subscriber may, at will, display any 
one of the signals belonging to his line, thus calling the attention 

378 



THE DIVIDED MULTIPLE SYSTEM. 379 

of one of the operators in the corresponding division. In order 
to enable the operator to make the connection with the calling line 
in response to such a signal, an answering jack is associated with 
each signal. This arrangement, therefore, necessitates the use of 
four line signals and four answering jacks for each line, one in each 
division of the board, and of means whereby the subscriber may dis- 
play any one of the signals belonging to his line to the exclusion 
of the others. Instead of trunking between exchanges, therefore, 
the subscriber sends his call directly into the division of the switch- 
board to which the desired subscriber belongs. 

Remembering that each division of the board has a multiple jack 
on each of its sections for every line belonging to that division, it 
will be seen that a subscriber may have his line connected with a 
line in any division by displaying his own signal on the board of 
the division to which the called-for subscriber belongs. The oper- 
ator at the position of that board at which the signal is thus dis- 
played may complete the connection between the answering jack 
of the calling line and the multiple jack of the called-for line, the 
latter line being provided with a multiple jack on every section of 
its own division of the board. 

By the use of the four-division board it is seen that the ultimate 
capacity of the switch-board at any office employing a given-sized 
jack may be increased approximately fourfold; not quite this, how- 
ever, because the extra line signals and answering jacks take up a 
portion of the room which would otherwise be available for the 
multiple jacks. 

To illustrate this more clearly the ordinary multiple board, using 
jacks placed on quarter-inch centers, may be made for a capacity 
of approximately 24,000 lines, this limit, of course, being reached 
because it is impossible to put more than 24,000 quarter-inch jacks 
within the reach of a single operator — that is, within the limits of 
a single switch-board section. By the use of the four-division sys- 
tem, however, using quarter-inch jacks, four multiple switch-boards, 
having a capacity of somewhat less than 24,000 lines each, could 
be used, the arrangement being such that any subscriber could se- 
cure a connection with any line without employing the services of 
more than one operator for any connection, and without any trunk- 
ing whatever between the boards. On account of the space occu- 
pied by the extra answering jacks and signals, the capacity of such 
a divided board would be somewhat under 96,000 lines. Such, in 
general, is the plan of the divided multiple board. 



380 



AMERICAN TELEPHONE PRACTICE. 



It is not necessary that the number of divisions be confined to 
four. The electrical and mechanical problem involved in the de- 
sign in such a board would be considerably simplified if two di- 
visions only were used; and it is also possible, at least theoretically, 
that the number of divisions might be increased to six or eight. 

Taking up the simplest form of divided multiple board — the two- 
division board — the principle of its operation will be readily under- 
stood from Fig. 294, where two lines, numbered A-101 and B-204, 
are shown entering the central office. In the divided multiple sys- 
tem it is obviously necessary to designate the division to which a 
line belongs, as well as its number in that division. The board 
shown consists of two divisions, A and B, each consisting of six 
sections of multiple switch-board. Line A-101 has an answering 
jack on section 1, and a drop or signal associated with this jack on 
the same section. This line also has a multiple jack, /, on each sec- 



eW"-S> 



Section 7. 



Seclian &. 



Dii/ision-Jl 
Section 3. Section 4-. 



Section 5. 



Section, if. 



Vr 



A-firop 



J3-&C4 



Secii m J 



rw 



u 



A^ 



w 



w 



Section &, 



Dii/l^ion-B. 

Section 3. f Section, &. 



Sectivn 5. 



Section i 






T7 






w 



W u\ 



FIG. 



294.-SCHEMATIC DIAGRAM OF DIVIDED MULTIPLE SWITCH-BOARD 
WITH TWO DIVISIONS. 



tion of this division. It also has a drop and an answering jack on 
section i of the B division of the board. 

By pressing either one or the other of two buttons on his tele- 
phone set any subscriber may throw the A or B signal on his line, 
thus attracting the attention of the operator in the A or B division 
of the switch-board. Thus, if subscriber, A-ioi, desires to con- 
verse with any other subscriber in the A division, he would press 
the A button, thus throwing his A drop at section I of the A di- 
vision. The operator would connect with this line at the A answer- 
ing jack, and upon learning the number of the A subscriber with 
whom he desired connection, would complete this connection by 
means of an ordinary pair of plugs and cords, the calling plug being 
inserted into the multiple jack of the A subscriber called for. If, 
on the other hand, subscriber, ^-101, desires to converse with some 
B subscriber, he would press his B button, which would throw his 
B drop, in this case on section I of division B. The operator at that 



THE DIVIDED MULTIPLE SYSTEM. 



381 



section would answer in the usual manner, and upon learning the 
number of the subscriber called (which, being a B subscriber, will 
have a multiple jack on each B section), she will complete the con- 
nection by inserting the calling plug into this multiple jack. Line 
B-204, has a multiple jack, /, on each section of the B division, an 
answering jack and drop on section 3 of the B division and on sec- 
tion 5 of the A division. 

In a similar manner a schematic representation of a four-division 
board is shown in Fig. 295, in which four lines, A-101, B-204, C-icg 
and D-1500 are shown, each having multiple jacks on the division 
bearing the same letter, and each also having an answering jack 
and drop on one section of each of the divisions. A subscriber de- 




Bit/isionA 



Section ?*. 



Section 3. 



Section 5. 






"P- 

?) C-ios 



\efack 



9 






Section & 



Section 3 



J$ii/ision-B 

Section 4- 



Section 5. 



Section 6. 



V 



np — 

Section 5. 



P — 

%\C-J09 

Section, 6 



Section & 



Dii/itiiori-C 

Section 3. Section 4-. 






Vr 



ff-isoo 



Dii/ipion-D 
Section 3 Section 



Vf 



Section 5. 



vr~ 






%C-jos 



FIG. 295.-SCHEMATIC DIAGRAM OF DIVIDED MULTIPLE SWITCH-BOARD 
WITH FOUR DIVISIONS. 

siring a connection with another subscriber will press the button 
on his telephone corresponding to the division to which the sub- 
scriber wanted belongs, which will throw the drop of the calling 
subscriber in the division of the called subscriber, and the operator 
will therefore make connection between the answering jack of the 
calling subscriber and the multiple jack of the called subscriber. 

The two notable examples of the use of the divided multiple sys- 
tem are to be found in the present main exchanges of the Kinloch 
Telephone Company, of St. Louis, Missouri, and that of the * Cuya- 
hoga Telephone Company, of Cleveland, Ohio. Each of these are 



* The divided multiple board at Cleveland has, since this writing-, been replaced by a 
straight multiple common battery board having an alternate capacity of IS. 000 lines. 



382 



AMERICAN TELEPHONE PRACTICE. 



four division boards, their present equipment consisting of 9600 
lines each, and their ultimate capacity being approximately 24,000 
lines each. These boards were installed before the wide adoption 
of common battery systems by the Independent telephone companies 
in the United States, and are not equipped with this desirable feat- 
ure. 

In Fig. 296 is shown a simplified circuit of a subscriber's line as 
used in these two exchanges. In the board at Cleveland the di- 
visions are given the names of the four letters, A, C, M and R, re- 
spectively, these letters being chosen instead of A, B, C and D, 
as used at St. Louis, to avoid the similarity in sound between the 
letters in the latter set, which had caused some confusion in the 





PlV. f^ 






l 



Tl 



H 



P1V. c. 



£fe 



S 



AHS. 



u 



is* 



rv 

no 



DIVA 



££ 



62 



FIG. 296.-LINE CIRCUIT OF DIVIDED MULTIPLE BOARD AT 
CLEVELAND, OHIO. 

earlier St. Louis exchange. The line circuit shown in Fig. 296 is 
that of an R line, as it will be seen that the R division is provided 
with multiple jacks, whereas the other divisions are provided with 
answering jacks. 

The tip and sleeve sides of the line are shown at the left-hand por- 
tion of the sketch, these terminating in two movable contact levers, 
I and 2, of the cut-off relay, R. These contact levers normally rest 
against their back contacts, 3 and 4, respectively, which connect the 
line normally with the signaling devices,?-, c, m and a } corresponding 
to the four divisions of the board, R, C, M and A. A simplified 
scheme of signaling is shown in Fig. 297. It represents the 
condition existing when the levers 1 and 2 of the relay R are in 



THE DIVIDED MULTIPLE SYSTEM. 383 

their normal positions. The four signaling devices, a, c, m and r, 
each consists of a polarized drop adapted to respond to current 
flowing in one direction only. 

The drops, a and c, are oppositely polarized and bridged directly 
across the tip and sleeve sides of the lines, T and S, respectively, the 
two drops being included in series. Similarly, the drops, r and m, 
oppositely polarized are bridged between the sleeve side of the line 
and ground, the two drops being in series. It therefore follows 
that a positive current sent over the metallic circuit of the line will 
actuate drop a only. A negative current over the metallic circuit 
will operate drop c only. A positive current sent over the line to 
ground will operate drop r, while a negative current over the same 




c - 



r+ 




m - 



FIG. 297.— SIMPLIFIED LINE SIGNALING CIRCUIT. 

path will operate the drop m only. The subscriber's apparatus is 
adapted, by means of four buttons, any one of which the subscriber 
may push, to send the currents in the proper direction either over the 
metallic circuit or over the sleeve side of the line and ground so as 
to operate any one of the drops in the manner thus described. 

Referring again to Fig. 296, it will be seen that the condition nor- 
mally existing with respect to the relation of the drops to the line, 
is that shown in simplified form in 297. 

Coming now to a consideration of the jack circuits, these are nor- 
mally entirely disconnected from the line, the connecting means be- 
tween them and the line being, the back contacts, 5 and 6, of the 
cut-off relay and the contact levers, 1 and 2, of that relay. Each 



384 



AMERICAN TELEPHONE PRACTICE. 



jack consists of a test ring, r, sleeve contact, s } tip contact, t, local 
contact, /_, and common ground contact, g. 

The plugs are of the two-conductor type, each adapted when in- 
serted into a jack to have its tip and sleeve register with the tip and 
sleeve contacts, t and s, of the jack. When fully inserted into the 
jack the sleeve of the plug does not touch the test ring, r. The in- 
sertion of the plug into the jack, besides completing the talking cir- 
cuit between the contacts, t and s, of the jack and those of the plug, 
presses the contact, i, belonging to that jack against the contact, g, 
thus closing the circuit traced from ground at the jack through the 
coil of the cut-off relay and the 20-volt battery back to ground. 
This actuates the cut-off relay, which by levers 1 and 2 breaks the 



AM5. 




FIG. 298.— CLEVELAND CORD CIRCUIT. 



circuit between the line and the various signaling devices, and com- 
pletes the circuit with the tip and sleeve contacts of the jack. The 
third lever, 7, on the cut-off relay serves to connect all of the test 
rings belonging to the line to ground through a resistance coil. 

It will be seen that the jacks belonging to this line are multipled 
throughout the various sections of division, R, of the board, 
while the jacks a', c' , m and /, which are answering jacks, are 
placed one on each division of the board, these jacks being, of 
course, located in close proximity to the corresponding drops. 

The cord circuit of this system is shown in Fig. 298. This is not 
adapted to common battery talking, but the answering and calling 
sides are divided by a repeating coil. Between the center points of 
each side of this coil and ground are connected the coils of the super- 



THE DIVIDED MULTIPLE SYSTEM. 



visory relays, 5* and S f , and the common battery. Each relay, when 
operated, serves to close the circuit of the corresponding- super- 
visory lamp, and it will be seen that as long as the circuit of the sub- 
scriber's instrument remains free from ground, as is the case during 
talking, the supervisory relays, ^ or S', of the plug connected to 
that line, will receive no current, therefore the lamp will not light. 




FIG. 



299.— CIRCUITS OF SUB-STATION APPARATUS FOR 
FOUR-DIVISION SYSTEM. 



When, however, the subscriber's receiver is hung up current flows 
from ground at the central office through the battery and the cor- 
responding supervisory relay, thence through one side of the corre- 
sponding repeating coil to the ground at the subscriber's instru- 
ment. The feature of double supervision is thus secured in accord- 
ance with the standard code of signaling. 

25 



386 AMERICAN TELEPHONE PRACTICE. 

In the earlier installation of this four-division system at St. Louis, 
calling was done by means of a magneto generator, having a com- 
mutator so arranged as to deliver either positively or negatively 
polarized current, instead of the usual alternating current. By 
means of four buttons current in the proper direction, either over 
the metallic circuit or over the sleeve side of the line and ground 
could be sent to the central office when the generator was operated, 
thus causing the operation of the drop at the division corresponding 
to the letter of the button pressed. At Cleveland, however, the 
magneto generator was dispensed with and the energy of the local 
battery at the subscriber's station used for signaling the central 
office. 

In Fig. 299 is shown diagrammatically the apparatus and cir- 
cuit arrangement of a sub-station equipment as used in the Cleve- 
land exchange. The local battery, B, used for signaling is the same 
one that is used for talking, and the only action necessary on the 
part of the subscriber to call central is a pressure of the button cor- 
responding to the division of the switch-board desired. The two- 
line wires are shown permanently attached to the two outside bind- 
ing posts, T and S, at the top of the figure, while the center post, G, 
is permanently connected to earth. The hook switch is of the Kel- 
logg type already described, and serves, when depressed, to connect 
the call bell, which is the ordinary type of polarized ringer, between 
the binding post, S, and ground. When raised the bell circuit is 
opened and the usual primary and secondary circuits are closed. 

A, C, M and R represent the call-sending devices, each one of 
which, when operated, is designed to actuate the corresponding 
annunciators, a, c, m and r, at the central office. Each one of these 
devices contains a bank of seven springs, lettered in their order from 
the top, a, b, c, d, e, f and g, the functions of which will be presently 
pointed out. Operating in connection with these springs is a closed 
magnetic circuit, induction coil, /, which has primary windings, p, 
and secondary windings, s. The structure of one of the sets of sig- 
naling springs and its mechanical and electrical operation in con- 
nection with the induction coil and the battery will be more readily 
understood by reference to Fig. 300, which shows the circuits 
stripped of confusing detail. Pressure is applied to the springs, b, 
of any one of the signal-sending devices by means of the push but- 
ton, K, not shown in Fig. 299, but shown in Fig. 300. When de- 
pressed the spring b depresses first the spring d into engagement 
with the spring e, and immediately thereafter spring f into engage- 



THE DIVIDED MULTIPLE SYSTEM. 



387 



ment with the spring g. Meanwhile the spring, c, also depressed by 
the spring b, has its forward end caught under the retarding spring 
a. When the spring c is thus caught by the spring a, it is held in 
its depressed condition as it has not sufficient strength in itself to 
overcome the retarding action of this spring. The spring b, how- 
ever, is the strongest of the group in its upward tendency, and when 
relieved of pressure from the button, K, engages with the upwardly 
bent portion of the spring c, and serves to drag it back to its normal 
position against the retarding influence of the spring a. When re- 
lieved from the influence of the spring a, the spring c assumes its 
normal position and breaks contact with the spring b. 

The cycle of contacts made and broken upon the pressure and 



a- 




FIG. 300.-DETAIL OF SUBSCRIBER'S SIGNALING APPARATUS. 



subsequent release of the button, K, is as follows: First, the making 
of contacts between the springs d and e ; second, the making of con- 
tacts between springs f and g ; then, as the button rises, first, the 
making of the contacts between the springs b and c ; second, the 
breaking of the contacts between the springs / and g; third, the 
breaking of the contacts between the springs d and e, and fourth, 
the breaking of the contacts between the springs b and c. 

These makes and breaks occur in a definite order, and with a pre- 
determined time interval between them, the purpose of which will 
be understood when the circuit connections are considered. 

The primary windings, p, of the induction coil, /, are connected 
in series with the battery between the springs f and g. The sec- 



388 AMERICAN TELEPHONE PRACTICE. 

ondary windings, s, of the induction coil are connected in series be- 
tween the springs b and e. The binding post, S, connecting with the 
sleeve side of the line is connected to the spring c. The spring d is 
connected to binding post T. The connections described and shown 
in Fig. 300 are those of an A button. 

Normally, therefore, both sides of the secondary and line circuit 
are open, as is also the primary circuit. When the button, K, is de- 
pressed, the secondary or line circuit is held open between the 
springs b and c as long as there is pressure on the button. The first 
contact that is made is that of one side of the secondary circuit be- 
tween the springs d and e. The flow of current through the primary 
winding of the induction coil made possible by the closure of springs 
/ and g would send an impulse to line but for the fact that the 
line circuit is held open between the springs b and c. In this manner 
the sending of the make impulse is avoided. When the pressure on 
the button, K, is relieved, the remaining side of the secondary or 
line circuit is closed between the springs b and c, thus placing the 
line in readiness to receive the break impulse of the induction coil, 
which immediately follows by the breaking of the primary circuit 
between the points / and g. It is this current which passes over the 
line to actuate one of the drops or annunciators, at the central office. 
Immediately afterward, both sides of the secondary circuit are 
opened and both sides of the secondary coils are thus left free for 
any subsequent connection. It will be noticed that the primary cir- 
cuit is kept closed during the change from the downward to the up- 
ward movement of the button. This change occupies a certain time 
interval, during which the current in the primary circuit has time 
to rise to its maximum strength in order to completely magnetize 
the core of the induction coil, so as to send a secondary current of 
maximum strength when the break occurs. The make impulse is 
eliminated, because it is weaker than the break, and the two being 
in opposite directions makes it impossible or impracticable to use 
both of them. The relation of the springs b and c effectually pre- 
vents the possibility of sending the make impulse over^the line, and 
therefore sending the wrong signal, because it is impossible to close 
the primary circuit between the points / and g without first breaking 
the secondary circuit between the contacts b and c. 

Reference to the circuits shown in Fig. 299 shows that the con- 
nections are such that the signal-sending devices A and C will send 
currents in opposite directions over the metallic circuit formed by 
the limbs, T and S, of the line, while the signal-sending devices, M 



THE DIVIDED MULTIPLE SYSTEM. 389 

and R, will send currents in opposite directions, over a circuit 
formed by the limb, S, of the line, with an earth return. The direc- 
tions in which these signaling currents are sent by each signal- 
sending device, A, C, M and R, correspond to that adapted to 
operate the respective annunciators, a, c, m and r, of Figs. 296 and 
297. 

The divided multiple system has never yet been extensively ap- 
plied to common battery systems, although the plan is entirely feas- 
ible, particularly with regard to a two-division system. The cir- 
cuits of the two-division system, adapted to both common battery 
talking and signaling, are shown in Fig. 301, their operation being 
as follows : The switch-board is shown as having two divisions, 
termed A and B. Two subscribers' lines are shown extending 
from their respective sub-stations, 1 and 2, to the central office. 

At the sub-station is provided the usual common battery equip- 
ment to which two push button keys, A and B, are added, one in 
each side of the line. Each push button is adapted, when depressed, 
to open its line conductor and ground it. , 

The line conductor, 1 , is permanently connected with the spring, 
3, of a relay, R, the normal contact, 4, of which is connected with 
one winding, 5, of relay, T, which winding is connected with the 
signal lamp, S, for the line, located upon the A division of the 
switch-board. The circuit is then extended from the other terminal 
of the lamp through the pilot relay, P, to the live pole of common 
battery, B 2 , the opposite pole of which is grounded. The other line 
conductor, 2, is permanently connected with the spring or movable 
contact, 6, of the relay, T, the normal contact, 7, of which is con- 
nected with one winding, 8, of the relay R, and thence to the signal 
lamp, S', of the line, located in the B division of the switch-board. 
The other terminal of the lamp is connected through the winding of 
the pilot relay P' ', to the battery B 2 . The relay R is provided with a 
second movable contact, 9, the normal contact, 10, of which is 
grounded, and the forward contact, 11, of which is connected to 
one terminal of the coil, 8, on this relay. This movable contact is 
joined to a similar movable contact, 12, of the relay, T. the normal 
contact, 13, of which is likewise grounded, and the forward contact. 
14, of which is similarly connected to one terminal of the coil, 5. on 
the relay, T. The front contact, 15, of the spring, 3, of relay R, of 
line wire, 1, is connected to the tip springs of the answering jacks, 
/ and /', in the A and B divisions, respectively, and the multiple 
jacks, J 2 , in the B division, while the front contact, 16, of the 



pr"- lime. \y— «s ^y. 




0) (* 
3 c 



>3i 

rooc# 



firi 



5 li " e o^R 



FIG. 301.— TWO-DIVISION COMMON-BATTERY SYSTEM. 
390 



THE DIVIDED MULTIPLE SYSTEM. 391 

spring, 6, of the relay, T, is similarly connected with the sleeve con- 
tacts of all the same jacks. The multiple jacks, J 2 , for this line are 
located upon the B division of the switch-board, this, therefore, be- 
ing a B line. The similar contacts of all the jacks belonging to any 
line are connected in multiple. The sleeve conductor leading to the 
jacks is also connected with one terminal of the winding, 17, of the 
relay, T, which winding is connected with a similar winding, 18, of 
relay, R, the other terminal of which latter winding is grounded. 
The second windings of these relays are, • therefore, connected in 
series between the sleeve contacts, all of the jacks, and ground. 

The pilot relay, P, located in the A division of the switch-board., 
controls, through its normally open contacts, the circuit of the pilo' 
lamp S 2 , this lamp being common to all lines of one position ; sim- 
ilarly, the pilot relay P', located upon the B division of the board, 
controls through its normally open contacts the pilot lamp S 3 . 

The line of station No. 2 is provided with exactly similar appa- 
ratus, except that the multiple jacks, J 2 , for this line are located upon 
the A division of the switch-board. This is therefore an A line. 

The cord circuit apparatus used alike at the various positions of 
both divisions of the switch-board is indicated in the diagram by 
the single set at the A division. The cord circuit will be recognized 
as being substantially the 'same as that used in the Kellogg single 
division board discussed in Chapter XX. 

The operation of the system is as follows : The calling sub- 
scriber, say on line 1, having ascertained the division upon which the 
wanted subscriber is listed in the directory of the exchange, would 
push the button corresponding to that division, the buttons being 
lettered in accordance with the names of the two divisions. If it be 
desired to operate the signal located upon the B division of the 
switch-board, the button B is depressed, thus grounding the line 
conductor, 2, and thereby completing a circuit from the live pole 
of the battery, B 2 , through the winding of pilot relay P', the line 
lamp S', winding, 8, of relay R, contacts, 7 and 6, of relay T, and 
over conductor, 2, to ground at the sub-station. The current in this 
path is sufficient to actuate the relay R, which locks by virtue of its 
armature, 9, closing the circuit through its coil, 8, to ground through 
the back contact of the spring, 6, of the relay T. This locking branch 
is in parallel with the line conductor, 2, and therefore replaces it in 
the further operation of this signal and relay. If the resistance of 
the line conductor is so great as to prevent the full operation of the 
lamp, S\ over the path first completed, the substitute path over the 



392 AMERICAN TELEPHONE PRACTICE. 

locking circuit being entirely local, is of low enough resistance, so 
that the current flowing brings the lamp up to full illumination and 
operates the pilot relay, P' . The pilot signal, S s , located before the 
operator upon the B division, is also lighted by current from the 
battery B 2 . 

If the subscriber desired had been listed in the A division, the call- 
ing subscriber would have depressed button A at his sub-station, 
thus operating relay T, which would lock up and light the pilot and 
line lamps, S 2 and S, respectively in the same manner as when the 
signals on the B division were caused to operate. 

In either case the relay, R or T, remains operated by virtue of its 
locking coil, thus holding the circuit of its corresponding lamp 
closed after its subscriber releases his button. Thus a subscriber 
may signal either division of the board. 

The operator attending the answering jack of the line, I, upon the 
division receiving the signal (say the A division), upon seeing the 
signals exposed inserts the answering plug of her cord circuit into 
the jack, /, and depresses the listening key to connect her telephone 
with the cord circuit, and receives the order from the subscriber. 
The insertion of the plug completes another path for current from 
the battery, B 2 , through the sleeve supervisory relay, R 3 , to the 
sleeve strand of the cord and the relay, T, and the winding 18 of 
relay, R, to ground, over which path current flows to actuate relays, 
R and T, as well as supervisory relay, R 3 . The actuation of the 
relay, R, breaks the locking circuit of the relay, T, at contact, 10, 
and therefore breaks the local circuit of the line lamp, S, and the 
pilot relay, P. The relay, T, is also actuated to prevent any pos- 
sible lighting of the line lamp, S', upon the other division of the 
switch-board. At the same time the spring, 3, of relay R and spring, 
6. of relay T engage their forward contacts, 15 and 16, and complete 
the metallic talking circuit from the line conductors, 1 and 2, to the 
tip and sleeve conductors leading to the jacks. If the line called 
for is idle, the plug is inserted in the proper multiple jack and the 
ringing key is actuated. The insertion of the plug closes a path 
for current from the battery, B', over the sleeve of the cord circuit 
to the winding, 17, of relay T, winding, 18, of relay R to ground, thus 
actuating both relays, and rendering the line signals of both di- 
visions of the board inoperative, and also connecting the spring-jack 
contacts of the line to the external line conductors. The operation 
of the ringing key does not effect any re-arrangement of the relays, 
R and T, for the reason that the sleeve side of the key closes a cir- 




393 



394 AMERICAN TELEPHONE PRACTICE. 

cuit from the battery, B 2 , through the resistance, 19, to the sleeve 
conductor of the jack. 

The operation of testing and of the supervisory signals, and, in 
fact, of the cord circuit as a whole, is the same as in the single di- 
vision two-wire system of the Kellogg Company. 

The part that the divided multiple board is to play in the future 
development of telephony is now the subject of much discussion. 
A few years ago there seemed to be a decided need for this system 
on account of the then present limit of the straight or single division 
multiple board to approximately 6000 lines. With the advent of 
the three-wire multiple board, as developed by the Western Electric 
Company, by which the size of the jacks are reduced to f inch be- 
tween centers, and the further simplification resulting in the pro- 
duction of the two-wire system, with its 3-10-inch or ^-inch jacks, 
the capacity of the single division multiple board has been increased 
to such an extent as to make it seem unnecessary to adopt the di- 
vided multiple system in central office equipments recently built, 
these not requiring a capacity above the present limit of the single 
division multiple board. 

The advantages of the single division multiple, when the question 
of ultimate capacity does not enter, are that its circuits and appa- 
ratus are simpler, and that there is no work required of the sub- 
scriber other than to remove his receiver from its hook and express 
his wants in words. The divided multiple necessitates the selec- 
tion between divisions on the part of the subscriber, and this, while 
in nowise fatal and perfectly practicable, is better obviated, if possi- 
ble. 

It must be said, on the other hand, that in a large office, say for 
18,000 lines, the number of jacks required by a four-division board 
would be very much less than that required by a single division 
board. To illustrate: assuming an average of 100 subscribers for 
each operator's position, the 18,000-line board would have 183 posi- 
tions, or sixty-one sections. The number of multiple jacks would, 
therefore, be 1,098,000, and with the 18,000 answering jacks added, 
we have a grand total of 1,116,000 jacks for the single division board. 
For the divided multiple, assuming four divisions, each division 
would be required to handle ztqoo lines. Each division would, there- 
fore, require 45 positions, which would be increased to 47 when the 
end positions were added, and to 48 in order to make the number 
of sections even. There would, therefore, be 16 sections in each 
division and 72,000 multiple jacks. Adding to this the 4500 an- 



THE DIVIDED MULTIPLE SYSTEM. 395 

swering jacks, we have a total for each division of 76,500 jacks, and 
for the four divisions a grand total of 306,000 jacks. We thus have 
for an 18,000 line board, 1,116,000 jacks for the straight multiple 
board, and 306,000 jacks for the divided multiple board, a saving 
of 810,000 jacks. This is not all clear gain, however, for while 
there would be 18,000 line signals for the straight multiple board 
there would be 72,000 for the four division board, which latter sys- 
tem would thus require 54,000 more line signals than the straight 
multiple board. Besides this, the four division board has more 
complex wiring and apparatus. 

If in the near future the increasing demands for telephone service 
make it desirable to install in a single office equipment for a greater 
number of lines than can be handled by a single multiple board, it 
will be necessary to use the divided multiple system unless some 
radically different means of handling connections than the manual 
board, as at present conceived, shall come into existence. 

Fig. 302 is a view of one of the divisions of the switch-board of 
the Cuyahoga Telephone Company at Cleveland. This board was 
installed by the Kellogg Company in 1899, and has a present equip- 
ment for 9600 lines. Its ultimate capacity is for about 22.000 lines. 



CHAPTER XXIII. 
PRIVATE BRANCH EXCHANGE SERVICE. 

By ''private branch exchange" is meant an exchange, complete 
in itself, in that it is adapted to bring into communication any two 
subscribers in a comparatively small community, such as that in a 
business establishment, and also affording communication between 
these subscribers and those of a larger exchange, such as that of 
a city. It is a "private" exchange because it is individual to the 
particular institution which it serves. It is a "branch" exchange 
because it operates in conjunction with and forms a part of a larger 
exchange, hence the term "private branch exchange." 

Of all classes of telephone service, the private branch exchange 
affords one of the greatest conveniences to the business man, and 
this fact has not been duly appreciated in the past. The telephone 
men and telephone subscribers are just awakening to a realization 
of its value. 

Methods employed in giving private branch exchange service 
differ widely among the various operating companies, and there 
appears to be no standard practice even among the Bell licensees, 
where the best organization usually exists. For this reason a study 
of the principal methods of handling this important branch of ser- 
vice will prove of interest. 

So far as the handling of local connections between two subscrib- 
ers in the same private branch exchange is concerned, there is little 
to be said, as the same conditions exist as in any small exchange. 
With respect to the method of connecting the private branch sub- 
scriber through a trunk line to a subscriber in the city exchange, 
or of connecting a subscriber in the city exchange to a private 
branch subscriber, there appears to be a wide diversity of opinion. 
Some contend that the function of the attendant at the private 
branch board is merely to secure for the private branch subscriber 
the proper connection, either incoming or outgoing, after which 
the connection is to be treated strictly as a main office connection 
in which the private branch attendant has no further concern. 
Another and stronger contention is that the private branch at- 
tendant's function is to, in every way in her power, serve the pri- 



PRIVATE BRANCH EXCHANGE SERVICE. 397 

vate branch subscriber, relieving him of waiting, as far as possible, 
performing for him all the duties necessary to secure the party with 
whom he wishes to speak, and supervise for him the disconnection 
of his line when through speaking, securing another connection 
if he so desires. At all events, however, the private branch attend- 
ant has a most important function in serving as a go-between for the 
general public and the private branch subscriber with respect to 
incoming calls. She receives all such calls and distributes them 
to the various departments of the institution she serves, in accord- 
ance with whatever intelligence she possesses. 

In nearly all cases a private branch exchange equipment consists 
of a number of sub-station equipments and of a switch-board of 
suitable size, from which lines extend to the various sub-station 
equipments of the branch exchange subscribers. The operation and 
arrangement of the exchange within itself is practically the same 
as that of any ordinary common battery exchange. 

Connections between a subscriber in a branch exchange and a 
subscriber in a city exchange are made in the same general manner 
as between two subscribers in different offices of a city exchange, 
i.he connection being through a trunk line extending between the 
two switch-boards. It is customary, however, in order not to con- 
fuse the work of the A operators in the city exchange, and on ac- 
count of economy in the trunk lines, to use two-way trunks in con- 
necting a private branch switch-board with a larger central office, 
instead of using one-way trunks, as has already been described in 
connecting two central offices in the same exchange. In other 
words, borrowing from the nomenclature of railroad systems, a 
single-track instead of a double-track system is used. 

As in other branches of telephone service, the tendency is to 
have the subscribers' or A operators at the multiple board of the 
city exchange perform no more special work than is absolutely 
necessary. For this reason private branch trunk lines usually ter- 
minate in the main office multiple switch-board in the same manner 
as regular subscribers' lines, the trunk lines being provided at the 
main office board with an answering jack and lamp at one section of 
the switch-board, and with a multiple jack on each of the 
sections. With this arrangement the work of the A operators at 
the main office is kept standard for handling incoming or outgoing 
calls over these trunk lines in exactly the same manner as over the 
regular subscribers' lines. The trunk lines may terminate at the 
private branch switch-board in different ways ; perhaps the most 



398 AMERICAN TELEPHONE PRACTICE. 

common being to terminate each in a plug and a magneto drop. In 
some of the latest systems, however, they terminate at the private 
branch end, each in a lamp and a jack. 

A private branch subscriber desiring a city connection usually 
simply removes his receiver from its hook, and when answered by 
the private branch attendant tells her to obtain for him a line of a 
certain number, or, as is frequently the case, not knowing the num- 
ber, tells her that he wishes to speak with Mr. Jones, of Smith & 
Co. He believes that the private branch attendant's time is less 
valuable than his own, and acts on that assumption, allowing her to 
do the work of looking up the number in the directory, of calling 
for the proper line in the main exchange, and of waiting for the 
response of that party. When the connection is secured she rings 
the bell of the private branch subscriber who called, who, if he has 
a proper regard for the ethics of telephone service, will remain at 
his desk so as to respond immediately, and thus avoid keeping Mr. 
Jones waiting. This is the usual method of handling an outgoing 
call from a private branch exchange, although there are some who 
contend that the work should be assumed by the subscriber making 
the call, the operator serving merely to connect the private branch 
subscriber's line with the proper trunk at the city office, after which 
the private branch subscriber would behave exactly as if he were 
on an individual line extending directly from his instrument to "the 
main office. 

It is difficult to see why the first method is not the proper one, 
for it cannot be denied that the operator's time costs less to the 
company employing her than does the time of those employes 
who are deemed of enough importance to be provided with a pri- 
vate telephone. Again, the economy in the time of the use of trunk 
lines is best secured by allowing the operator to attend to the mak- 
ing up of the connection, and as the charge for trunk lines usually 
forms the most important item of cost in private branch exchange 
service, this matter is of importance. In this connection the pri- 
vate branch attendant may perform the important function of super- 
vising the use of the trunk lines, refusing to allow minor employes, 
such as stenographers, to gossip or to conduct private business over 
the line, when they are employed for more important purposes. 

With regard to incoming calls for the institution served by the 
private branch exchange, the work of the attendant is of even more 
importance. Learning the nature of the business of a calling party, 
she may connect him with the proper department or official without 



PRIVATE BRANCH EXCHANGE SERVICE. 399 

causing him the annoyance of being referred from one department 
to another. This also relieves the private branch subscriber of the 
annoyance of being continually called up by persons whose busi- 
ness should be transacted with some one else. In the case of the 
higher officials of a company, this duty of the private branch attend- 
ant is of particular importance, she acting as a doorkeeper, as it 
were, allowing only those who have proper business with the official 
to gain access to him by telephone. 

Assuming that a connection between two subscribers, one in the 
private branch exchange and one in the city exchange, has been 
made, the method of supervising this connection is the subject of 
much discussion. Of course, the city subscriber will always have 
a supervisory lamp at an A operator's position, at the city exchange, 
under his control, and the supervision of his line will, therefore, be 
the same as in any other connection. There are three general 
methods, however, of conveying the supervisory signal from the 
private branch line, which may be briefly outlined as follows: 

The first of these is to place in the control of the private branch 
subscriber during a connection the regular cord circuit supervisory 
signal at the main office only, so that this lamp will light at the main 
office when the subscriber hangs up his receiver, in exactly the same 
manner as if he were on a* regular subscriber's line entering the 
main office directly. With this method in use, when both subscrib- 
ers hang up their receivers, the main office operator would pull 
down the connection, and in doing so the removal of her plug from 
the private branch exchange trunk jack would light the disconnect 
lamp at the private branch board, after which the private branch 
attendant would pull down the connection. 

In the second of the three methods, the private branch exchange 
subscriber has within his control during a connection a lamp at 
the private branch board only, the action of his hook causing no 
change whatever at the main office. With this arrangement the dis- 
connection is effected when both subscribers hang up, the private 
branch exchange operator first pulling down the connection in re- 
sponse to the lighting of the lamp under the control of the private 
branch subscriber (she receiving no signal from the city subscriber) 
whereupon she pulls down the trunk connection, thus lighting the 
supervisory lamp at the main office. The main office subscriber hav- 
ing also hung up, the subscriber's operator, who made the connection, 
seeing both lamps lighted, will then pull down the connection. It 
will be seen that the operation at the main office is rendered in no- 



400 AMERICAX TELEPHOXE PRACTICE. 

wise special, the subscribers' or A operator, receiving both super- 
visory signals in exactly the same manner as if two local lines had 
been connected. The only difference is that instead of both super- 
visory lamps being operated directly by the movement of the con- 
nected subscribers' hooks, one of them is operated by the action of 
the private branch exchange operator in pulling down the connec- 
tion, 

In the third method, which is, perhaps, the most used of all, the 
private branch subscriber has within his control the supervisory 
signals at both the private branch exchange and the main office, so 
that when he hangs up his receiver both lamps light, thus send- 
ing a disconnect signal simultaneously to the main office and to the 
private branch office. 

From the standpoint of clean operating at the main board, the 
first of these methods, wherein the private branch subscriber con- 
trols the supervisory signal at the main office only, is perhaps the 
best. It is found, however, that this arrangement is not desir- 
able from the standpoint of the private branch subscriber, and also 
has its disadvantages to the city subscriber. To illustrate this : A 
private branch subscriber will often, when called up by a city sub- 
scriber, wish to direct the calling city subscriber to some other 
member of the firm or some other department where his business 
may be best transacted. Under this circumstance the private branch 
subscriber is not able to signal the branch exchange attendant, which 
he might do if he controlled the supervisory signal at the private 
branch board ; but on the other hand, he is obliged to tell the city 
subscriber to call up on another line in the private branch ex- 
change. This necessitates the calling party beginning all over again 
at the main office, and perhaps thereby temporarily losing the use 
of the trunk line, over which the connection was first established. 

This fault is eliminated in the second of the three methods in 
which the private branch subscriber has within his control during 
a connection a lamp at the private branch board only. With this 
arrangement, the private branch subscriber who has been called, and 
who desires to have the calling party put in connection with another 
person on another private branch line, has only to move his hook 
up and down to signal his own private branch attendant, whereupon 
she will complete the connection between the calling subscriber and 
whatever other private branch line is designated. 

Another advantage of this second system of supervisory signal- 
ing is that if the private branch exchange subscriber desires another 



PRIVATE BRANCH EXCHANGE SERVICE. 401 

connection after the close of a conversation he can at once secure the 
attention of the private branch attendant and have her secure it for 
him, hanging up his own receiver until notified by the attendant 
that the connection is ready. This system has the disadvantage, 
however, of requiring a somewhat more complex cord and trunk 
circuit at the private branch exchange. 

Under the third method, where the private branch subscriber con- 
trols the supervisory signal at both the private branch and the main 
board, some confusion is liable to occur by both operators coming 
in on the circuit at the same time in response, for instance, to the 
private branch subscriber's action of moving his hook up and down. 
In doing this he usually desires the attention of the private branch 
attendant, rather than of the main office operator. The main office 
operator, however, may notice this at once and cut her telephone 
out of the circuit. If the private branch subscriber desires to have 



lime: 



■i 



3t_o 



^m 






FIG. 303.-PRIVATE-BRANCH EXCHANGE SUBSCRIBERS' LINE CIRCUIT- 
WESTERN ELECTRIC SYSTEM. 

the party with whom he is connected switched on to another private 
line the main office operator is not likely to pull down the connection 
in response to the movement of his hook, because the main office sub- 
scriber will not have hung up his receiver, as he will be waiting for 
the new connection. 

This method of having the signals at both the main office and the 
private office under the control of the private branch subscriber has 
the advantage of simplicity and first cost, the circuit being very 
simple. It has a further advantage in common with the first 
method, of tending to clear the main office cord circuits more 
quickly after a connection than is the case where the second method 
is used. 

The third method is that now most used in the United States, 
probably on account of its simplicity and the consequent advantage 
in point of maintenance of the private branch exchange apparatus. 



40: 



AMERICAN TELEPHONE PRACTICE. 



Disregarding the questions of simplicity and cost, the second 
method of handling supervisory signals is probably the best from 
the standpoint of the operators and of the subscribers. 

Coming now to the consideration of the actual circuits and appa- 
ratus used in private branch exchange work, attention will first be 



LINE 




FIG. 304.— PRIVATE-BRANCH EXCHANGE TRUNK LINE CIRCUIT- 
WESTERN ELECTRIC SYSTEM. 

called to a system now in use by many of the Bell operating com- 
panies. 

The circuit of such a private branch exchange line is shown in 
Fig. 303, the circuit of a trunk line leading to a main office being 
shown in Fig. 304. In the subscriber's line the sleeve contact of the 



LINE 



i 



f 31 



ZA 



Un 



i-.L 



X 1 



FIG. 305.— PRIVATE-BRANCH EXCHANGE SUBSCRIBERS' LINE CIRCUIT. 

jack is left open, while in the trunk line it is grounded. In this line 
circuit a self-restoring electro mechanical signal is used. Some- 
times, however, lamp signals are used, in which case a line relay is 
substituted for the mechanical signal, this controlling the circuit of 
the lamp, the connection then being that shown in Fig 305. 



PRIVATE BRANCH EXCHANGE SERVICE. 



403 



A cord circuit often used in connection with these line circuits is 
shown in Fig. 306. It will be seen that this is practically the same 
as the Western Electric common battery cord circuit using mechan- 
ical supervisory signals, instead of lamps controlled by relays, with 
the exception that two relays, A and B, are added. When the re- 
lays, A and B, are de-energized, as they are normally, the talking 
circuit from the answering plug to the calling plug embraces the 
windings of the usual common battery repeating coil between the 
center points of which is bridged the private branch storage battery. 
When, however, the relays A and B are actuated the repeating coil 
is cut out entirely, the tip of the answering cord being then con- 
nected directly to the tip of the calling cord, while the sleeve of the 
answering cord is connected to the sleeve of the calling cord through 
the coils of the supervisory signals, 5 and S'. In this case the bat- 




tle. 306.-WESTERN ELECTRIC CORD CIRCUIT FOR PRIVATE BRANCH 

EXCHANGE. 

tery has no connection whatever with the talking circuit. The coils 
of the relays A and B are connected in series between the live side 
of the battery and the third contacts on the answering and calling 
cords, these third contacts being permanently connected. The rea- 
son for grounding the sleeve contact on the trunk jack, and leaving 
it ungrounded on the private branch subscribers' lines, will now be 
apparent. When a pair of cords and plugs is used to connect two 
private branch subscribers' lines, the relays A and B will not be ener- 
gized because the circuit through them is not complete^ to ground 
at the ring of the jack. Under this circumstance the current for 
operating the subscribers' transmitters, and for operating the super- 
visory signals is drawn from the battery bridged across the cord cir- 
cuit. If, however, the private branch subscriber's line is connected 
through the trunk line to the main exchange both relays, A and B, 
of the cord circuit will be energized, because the circuit through them 



'MINIM 




JmTcry ) 



111 2 



4-n~n 




«tej#z 



*-* V J I II I I II 

T7j=^ — 

Wr MaaAi-im.ih-WW^ 




FIG. 307.— CONNECTION, PRIVATE BRANCH TO CITY SUBSCRIBER- 
WESTERN ELECTRIC SYSTEM. 
404 



PRIVATE BRANCH EXCHANGE SERVICE. 405 

is completed to ground through the ring contact of the trunk jack. 
In this case the battery and repeating coil are automatically cut out, 
and the private branch subscriber draws his battery for talking and 
for the operation of the supervisory signals directly from the main 
office battery, his line in this case being merely an extension of the 
trunk line. 

A connection between a private branch subscriber's line and a sub- 
scriber in the main office is shown in Fig. 307, the main office sub- 
scriber's apparatus being at the right of the figure. Assuming that 
the private branch subscriber originated the call, and requested sim- 
ply that his line be connected with the main office, the private branch 
operator would connect him with the main office switch-board by in- 
serting her calling plug in the trunk jack, which, by the ground upon 
the trunk jack, would operate relays A and B and would call the 
main office operator by reason of the flow of current from the main 
office battery through the line relay, and thence through trunk, pri- 
vate branch cord circuit, private branch line and private branch tele- 
phone, returning through private branch line, private branch cord 
circuit and trunk, to ground at main office. The main office opera- 
tor then handles the call exactly as she would any call, answering 
with her answering plug and completing the connection with her 
calling plug, and obeying her supervisory signals as for any two 
connected lines. The operation of the relays A and B of the private 
branch cord circuit would release the connection entirely from the 
battery at the private branch switch-board, and current from the main 
office battery would flow over the ring-side of the trunk line to the 
main office. When, therefore, the private branch subscriber hangs 
up his receiver or moves his hook up and down, desiring another 
connection, the supervisory signals at both switch-boards are oper- 
ated. It is seen, therefore, that this circuit employs the third method 
of supervisory signaling outlined in the beginning of this chapter. 

The connection between a main office and a private branch sub- 
scriber which originated with the main office subscriber would em- 
ploy the same apparatus, as shown in Fig. 307, but it would be estab- 
lished, of course, in reverse order. The main office operator, having 
received and answered the main office subscriber's call in the usual 
way, with her answering plug, would complete the connection with 
the multiple jack of the trunk line leading to the private branch ex- 
change in the same manner as if it were an ordinary subscriber's 
line, using her calling plug in the trunk jack and ringing as in ring- 
ing a called subscriber, but in this case the ringing would cause the 



406 



AMERICAN TELEPHONE PRACTICE. 



operation of the trunk drop at the private branch switch-board. • The 
private branch attendant, seeing this signal, would plug in with the 
answering plug and answer in the name or number of the firm to 
which the private branch exchange belonged. The calling sub- 
scriber then would tell her the nature of his business, or, perhaps, 
the person to whom he wished to speak, whereupon the attendant 
would complete the connection with the proper private branch line, 
using her calling plug, and by the operation of her ringing key would 
call the subscriber on that line. The operation of the relays A and 
B would remove the private branch battery from the cord circuit, 
and, therefore, when the private branch subscriber responded, three 
supervisory signals would be operated. In the operation of this cir- 
cuit, when the call is originated by the main office subscriber, the 
answering of the private branch attendant actuates the supervisory 




FIG. 308.-PRIVATE-BRANCH EXCHANGE LINE CIRCUIT OF CHICAGO 
TELEPHONE COMPANY. 

circuits upon the calling plug at the main office to show the main 
office operator that the call has been responded to, and that no fur- 
ther action at the main office is required. 

The main office operator performs exactly the same function in 
connecting with the private branch trunk line as she does in con- 
necting with a regular subscriber's line. 

It might be said that the compelling of the private branch opera- 
tor to restore manually the trunk line drop is a disadvantage, but 
this is hardly a valid objection, as the private branch attendant has 
very much more to do in the way of talking and listening than she 
has with her hands. 

In Fig. 308 is shown a line circuit of the latest private branch 
exchanges used by the Chicago Telephone Company. This em- 
ploys a jack which cuts off both sides of the line, and also has three 
additional spring contacts used entirely for modifying the opera- 



PRIVATE BRANCH EXCHANGE SERVICE. 407 

tion of the signals. These three additional contacts comprise a 
spring, a, normally resting against spring b, but pressed into con- 
tact with spring c, when a plug is inserted in the jack. When the 
circuit of the line is completed at the subscriber's station by the re- 
moval of the receiver from its hook, the current flows from the 
grounded side of the battery through the tip side of the line to the 
subscriber's station, thence over the sleeve side of the line through 
the coil of relay, A, to the live side of the battery. Both sides of the 
line are broken at the jack when the plug is inserted, but it will be 
noticed that the coil of relay, A, is left outside of the jack, so as to 
be in the talking circuit. The operation of the relay, A, in response 
to a call, completes the local circuit of that relay, which may be 
traced from the live side of the battery through a pilot relay, B, 
and the line lamp to the front contact of the relay, A, thence to con- 
tact b of the jack, and to ground through contact, a. This illum- 
inates the lamp and actuates a buzzer or night alarm on account of 
the action of the pilot relay B. Plugging into the jack does not de- 
energize the relay, A, as the talking current from the cord circuit 
passes through it. It will, however, put out the line lamp, by break- 
ing the contact between the springs a and b of the jack. After the 
plug is inserted the spring a of the jack makes contact with the 
spring c, and therefore the back of contact of the relay, a, is 
grounded, which means that when the armature of the relay A, is 
again released, which will take place when the subscriber hangs up 
his receiver, the line lamp will again be lighted, the circuit this time 
being from the live side of the battery through the pilot relay and 
lamp through the back contact of the relay A, and to ground through 
the contacts a and c of the jack. By this means the line lamp is 
made to serve also as a supervisory lamp, and for that reason no 
lamps or relays whatever are provided in connection with the cord 
circuits. This double use of the line lamp is made possible by the 
alternate positions of the spring a in the jack, as governed by the 
insertion of the plug. The front contact of the relay, A, is grounded 
when the plug is not inserted in the jack, while the back contact of 
the relay is grounded when the plug is inserted in the jack. This 
means that the front contact of the relay is adapted to complete the 
circuit of the lamp, while no plug is in the jack; and that after a 
plug is inserted the back contact of the relay controls the lamp. 

The pilot relay, B, controls a buzzer to serve as a night alarm, or 
to attract the attention of the operator if she is not at the board dur- 
ing the day. This buzzer is controlled by the key, K, which is really 



408 



AMERICAN TELEPHONE PRACTICE. 



an ordinary ringing key, its two inside contacts being left dead, 
while its outside contacts are wired in multiple, thus making the key 
serve as an ordinary single-tlirow switch. A 50-ohm resistance is 
wired in series with the buzzer, and a like resistance in multiple 
with it, this being for the purpose of reducing the amount of current 
that would flow through the buzzer from the storage battery, and 
also to reduce to some extent the noise which might be thrown on 
any connected lines by the operation of the buzzer. 

In Fig. 309 is shown the cord circuit adapted to use with the line 
circuit shown in Fig. 308. In the left-hand portion of this figure 
is shown a key, K', which, when in its normal position, connects the 
common battery between the two sides of the cord circuit, a retarda- 
tion coil, R or R f , being placed between each terminal of the bat- 
tery and the corresponding cord conductor. When this key is 




FIG. 309.-PRIVATE-BRANCH EXCHANGE CORD CIRCUIT OF CHICAGO 
TELEPHONE COMPANY. 

thrown the battery is entirely cut off from the cord circuit. At 
the right of the figure in connection with the calling plug is placed 
an ordinary combined ringing and listening key, which needs no 
explanation. 

This cord circuit, when used to connect two of the line circuits 
shown in Fig. 308, supplies battery current to each of the connected 
lines, the key, K', being left in its normal position so as to connect the 
battery across the cords. The retardation coils connected between 
the terminals of the battery and the strands of the cord serve to 
prevent cross-talk in the usual manner. 

The trunk circuit used by the Chicago Telephone Company, 
adapted to work in connection with the line circuit of Fig. 308 and 
the cord circuit of Fig. 309, is shown in Fig. 310. In this figure 
that portion of the circuit at the right of the vertical dotted line 
shows the arrangement at the private branch end, that at the left 



PRIVATE BRANCH EXCHANGE SERVICE. 409 

being the arrangement at the main office end of the trunk line. 
The battery A is therefore the common battery of the main ex- 
change, while battery B is that of the private branch exchange. It 
will be seen that the main office equipment is nearly the same as 
that on an ordinary subscriber's line, consisting of an answering 
jack at one section, and a multiple jack at each section, together 
with the usual line relay and line signal and cut-off relay. The line 
relay, P, is, however, connected with the grounded side of the bat- 
tery instead of the live side, as is usual. In the tip side of the line 
between the battery and the cut-off relay, C, a resistance, R, is 
placed. At the private branch end this trunk equipment consists 
of the same type of jack, /, as is used for the ordinary line jacks, as 
shown in Fig. 308. Besides this, a locking relay, L, controlling 
the trunk line signal, is provided, as is also a condenser, c, and a 
key, K". 

The operation of this trunk circuit is as follows: Assuming that 
a main office subscriber calls for a private branch subscriber, the 
connection of the calling subscriber with the main office end of the 
trunk line will be completed in exactly the same manner as in the 
case of connecting two local subscribers at the main board. When 
the main office operator inserts the calling plug into the jack of 
the trunk line no effect will be produced at the private branch ex- 
change on the relay, L, because of the presence of the condenser, 
C, which prevents direct current from passing through the coil of 
the relay. The main office operator must, therefore, ring on the 
trunk line in the same manner as if she were calling a subscriber, 
and the alternating current thus sent to line passes over the ring 
side of the trunk and the condenser, r, and thence to ground through 
the springs, a and b, of the jack and the battery, B. The operation 
of the relay, L, caused by the passage of this alternating current, 
will close contacts at d and e. The closure of contact at d will com- 
plete the circuit from the branch exchange battery through the 
springs, a and b, and the coil of the relay, L, to ground through the 
contact at d. This will lock the relay and hold it locked until the 
private branch exchange attendant plugs in. The closure of the 
contact at c by the relay, L, will light the trunk line lamp as a signal 
to the attendant. Meanwhile, the supervisory lamp of the calling 
plug at the main exchange remains lighted because the presence 
of the condenser, r, at the private branch exchange allows no direct 
current to pass over the trunk line to operate the calling supervisory 
relav. 



410 AMERICAN TELEPHONE PRACTICE. 

The private branch attendant, seeing the trunk signal illuminated, 
plugs in with one of the plugs of the cord circuit of Fig. 309, and 
at the same time throws the key, K', of that cord circuit so as to 
cut off the local battery from the cord circuit. She thus gets all 
battery current for talking from the central office, this current being 
supplied over the metallic circuit of the trunk line from the cord 
circuit of the A operator. Throwing the listening key of the pri- 
vate branch exchange cord circuit will allow current to flow through 
the operator's talking circuit over the metallic circuit of the trunk 
line, and will extinguish the calling supervisory signal at the main 
office; the main office operator will, therefore, pay no more atten- 
tion to the connection, seeing both lamps out, and the private branch 
attendant will find out from the calling subscriber the private branch 
line he desires. She will then complete the connection by plugging 
in with the calling plug and ringing. While waiting for the called 
subscriber to respond, the operator, if she keeps her listening key 
thrown, will prevent the relighting of the calling supervisory signal 
at the main ofrice. She will know when the private branch sub- 
scriber responds by the going out of the line lamp, belonging to 
that subscriber, which was lighted when she plugged into his line. 
When the private branch subscriber responds the connection is in 
proper condition for conversation, the private branch subscriber 
drawing his current for talking from the main office battery. When 
he hangs up his receiver the flow of current over his line and the 
trunk line will cease, therefore the calling supervisory relay at the 
main office will be de-energized, as will also the relay, A, Fig. 308, 
belonging to the private branch subscriber's line. This will light 
the calling supervisory lamp at the main office and the line lamp 
at the private branch board. 

The purpose of the key, K" , in the trunk circuit at the private 
branch board, is to allow the private branch attendant to break the 
circuit of the ring side of the trunk line, if for any reason she desires 
to signal the main office operator during a connection. Alternately 
pressing and releasing this key will cause the flashing of the super- 
visory lamp of the cord used in making the connection with the 
trunk line at the main office, thus giving the private branch at- 
tendant the same power of flashing the supervisory signal on the 
line lamp as is ordinarily held by a subscriber on a regular line. 

It is evident that when the attendant plugs into the jack, /, the 
local circuit through the relay, L, will be broken between contacts 
a and b of the jack, thus restoring this relay to its normal position 



PRIVATE BRANCH EXCHANGE SERVICE. 



411 



and putting out the line lamp, as well as breaking the ground con- 
nection at the contact, d. This relay cannot, therefore, be again 
operated except by ringing current from the central office. 

In establishing a connection in a reverse direction — that is, in 
response to the private branch subscriber making a call, the attend- 
ant at the private branch exchange may call central by plugging 
into the jack and throwing her listening key, this act allowing the 
flow of current over the trunk line and operating the line relay be- 
longing to the trunk line. The call will then be answered by the 
main office operator in exactly the same manner as if it were that 
of an ordinary line subscriber. 

It has been previously stated that the battery at the private branch 
office is frequently charged from the main office battery over one 




IMr 



FIG. 310.— PRIVATE-BRANCH EXCHANGE TRUNK CIRCUIT OF CHICAGO 
TELEPHONE COMPANY. 



or more of the trunk lines when idle. This is accomplished in the 
trunk line circuit shown in Fig. 310, it being obvious that when the 
trunk line is not in use a flow of current will exist from the live side 
of the battery, A, at the central office over the tip side of the trunk- 
line to the tip contact of the jack, /, at the private branch exchange 
to the ungrounded pole of the battery, B, at the private branch ex- 
change, and thence to ground. In order that there may be suffi- 
cient flow of current over this circuit, the battery at the private 
branch exchange is usually made to consist of a fewer number of 
cells than that af. the main office. Eight cells are frequently used 
at the branch exchange, while 1 1 are commonly used at the main 
office. The proper pole of the battery at the private branch exchange 
is grounded, it being necessary to ground the same pole as is 



412 



AMERICAN TELEPHONE PRACTICE. 



grounded in the main office. With the batteries so connected the 
flow of current through the private branch battery will be in the 
proper direction to charge it. A resistance coil, R, is placed in the 
lead which feeds the trunk line circuit at the main office, this 
resistance being so proportioned as to allow the proper amount of 
current to pass to keep the private branch battery fully charged 

Sometimes in private branch exchange work it becomes desirable 
to use party lines and to afford means for the different parties on 
the line to call each other without calling the central office, or to 
call the central office without calling the other stations on the line. 
This is often desirable in large institutions covering considerable 
territory. In giving such service as this in connection with mod- 
ern exchange systems, it is customary while supplying all current 
for talking purposes from a common battery to resort to manual 





L. 



l&^r° 



FIG. 311.— PRIVATE-BRANCH EXCHANGE PARTY LINE CIRCUIT— CHICAGO 
TELEPHONE COMPANY. 



calling between the various stations on the line. Thus each station 
is provided with the usual common battery talking and call-receiving 
apparatus, and with a magneto generator adapted to be bridged 
across the line when operated. In order to prevent the line signal 
at the private branch exchange being operated by current from the 
line generator, a differential line relay may be used, having one 
winding in each limb of the line. Any current sent over the me- 
tallic circuit of the line will, therefore, not operate this signal, and 
thus any subscriber on the line is enabled to call another the same 
as on an ordinary bridging bell line. In order to enable any sub- 
scriber to call the private branch office a push button is provided 
which grounds the live side of the line, thus causing the operation 
of the line relay and the lighting of the signal. 

Such a line circuit as used by the Chicago Telephone Company 



PRIVATE BRANCH EXCHANGE SERVICE. 413 

in connection with the private branch exchange circuits, shown 
in Figs. 308, 309 and 310, is shown in Fig. 311, this being the cir- 
cuit extending from the private branch board to several party line 
stations. Battery is normally fed from the private branch office 
through two retardation coils and through the two windings of 
the differential line relay to the two sides of the line. By this means 
any two subscribers on the same line may talk, both their instru- 
ments drawing battery current from the private branch office. 
This flow of current will not operate the differential line relay. The 
operation of any subscriber's generator will likewise not influence 
the differential line relay, but will ring all of the bells on the line 
in the same manner as in the ordinary bridging bell system, and 
therefore a code of long and short rings is used to enable the sub- 
scribers to call a certain party on the line. 

By pressing the key, K 3 , at any station the ring side of the party 
line is grounded, thus allowing the flow over one coil only of the 
differential relay, which will cause the action of that relay to attract 
the attention of the private branch attendant. The armature of 
this relay will, however, fall back as soon as the subscriber releases 
the pressure on the key, but when the operator plugs into the circuit 
the tip-wire winding of the differential line relay will be cut off from 
the talking conductors. Therefore the connection of this line with 
another private branch line or with the trunk line leading to the 
main office is the same as that described in an ordinary private 
branch line, as shown in Fig. 308. 

It is sometimes necessary for very large institutions to be pro- 
vided with two private branch exchange switch-boards. This is so 
where the same firm has different buildings in different parts of the 
city. Where this is the case it is frequently desirable to connect 
the two private branch offices directly by trunk lines in order to 
avoid the necessity of having calls for connections between sub- 
scribers in the two branch exchanges handled through the 
main offices. Where the two branch exchanges are very large this 
trunking between them could be accomplished by the double-track 
system, using two sets of one-way trunks. In most cases of this 
kind, however, a single-track system using two-way trunks is bet- 
ter on account of economy in the number of trunk lines required. 
Such a two-way trunk is shown in Fig. 312, this being adapted to 
use with the private branch circuits shown in Figs. 308 to 311. in- 
clusive. The equipment at each end of this trunk line is the same. 
and when the operator at one end plugs into the jack of the trunk 



414 



AMERICAN TELEPHONE PRACTICE. 



line the two trunk lamps, one at each end, light simultaneously. 
Both lamps are extinguished when the operator at the other end 
plugs in, in answer to a call. The first operator is, therefore, made 
aware of the response of the operator at the other end of the line. 
As soon as either operator pulls down the connection at one end 
both lamps will light, and both are extinguished when the other 
operator pulls down the connection. 

The method of operation by which these results are brought 
about may be clearly seen in Fig. 312. Two trunk line relays are 
provided, one at each end of the line, each being in a different side 
of the line. When an operator plugs in at, say, the left-hand end 
of the line, with a cord circuit such as shown in Fig. 309, the ring 





FIG. 312, 



-TWO-WAY TRUNK CIRCUIT BETWEEN TWO PRIVATE-BRANCH 
SWITCH-BOARDS— CHICAGO TELEPHONE COMPANY. 



spring of the jack will cause the spring, a, at that jack to be pressed 
into contact with the spring c, and thus light the lamp at that end 
of the line, the relay at that end of the line remaining inactive. The 
insertion of the plug will also ground the tip spring of the jack 
through the cord circuit at that end, thus allowing the flow of bat- 
tery to take place from the battery at the second office through a 
relay at the second office to ground at the first office. As the plug 
has not been inserted into the jack at the second office the attrac- 
tion of the armature of this relay will cause the lighting of the 
lamp at that office. When the operator at the second office re- 
sponds, the opening of the contacts, a b, in the jack at that office 
will cause the going out of the lamp at that office, while the ener- 
gization of the relay at the first office caused by the flow of cur- 



*m 



eM 



<j 



PR1 PR. EXCH. LIME 



n 



1 



Masai * 



fl 







±i= 



fl 








FIG. 313.— CONNECTION BETWEEN PRIVATE-BRANCH EXCHANGE AND 
CITY SUBSCRIBERS WHEREIN THE PRIVATE-BRANCH SUBSCRIBER 
CONTROLS THE SUPERVISORY SIGNALS AT THE PRIVATE-BRANCH 
BOARD. 

415 



416 AMERICAN TELEPHONE PRACTICE. 

rent over the line will cause the going out of the lamp at the first 
office. It is evident that when either operator withdraws her plug 
both lamps will light, and that both will go out when both operators 
withdraw their plugs. By virtue of the transposal of the connec- 
tions between the two sides of the trunk line and the trunk-line 
jacks, the tip of one jack being connected with the ring of the other, 
current from the two batteries of the cord circuits at the two ex- 
changes will thus flow in series over the trunk line. 

The question of using the lamp associated with the line, as both 
line and supervisory lamp, as is done in the private branch exchange 
system of the Chicago Telephone Company, is of considerable in- 
terest, and there is no reason, from an operating standpoint, why 
this should not be considered good practice in small boards in gen- 
eral. Of course it would be impracticable in a multiple board, be- 
cause one plug of a circuit used in connecting two lines would always 
be inserted in the multiple jack. As this jack would have no signal 
adjacent to it no supervisory signal would be provided. This 
scheme, as applied to small boards, has the distinct advantage of 
greatly simplifying the cord circuit, which, being devoid of all relays 
and signals, is reduced to its simplest possible form. The only thing 
that approaches complication about this system is the jack, which, 
consisting, as it does, of seven movable spring contacts, is somewhat 
objectionable. Considering, however, that there are only a few of 
them used, and that there is unlimited space in which to install them, 
there is no reason why such a jack should not be made to give per- 
fectly satisfactory operation. 

A system wherein the second method of supervisory signaling is 
employed will now be considered. In this it will be remembered that 
the supervisory signal at the private branch exchange board, and not 
that at the central office, is under the control of the private branch 
exchange subscriber. 

A private branch line circuit is shown at the left of Fig. 313. The 
signal in this is of the mechanical or gridiron type, already discussed 
in a previous chapter. 

This line is shown connected with a regular exchange line at the 
right of the figure, the connection being made through the cord cir- 
cuit at the branch exchange, the private branch trunk, and the regu- 
lar A operator's cord circuit at the main exchange. The arrange- 
ment at the main exchange end of this trunk is that of the standard 
Kellogg line circuit already described. At the private branch end an 
ordinary ring-down drop is used for the signal, this being bridged 




FIG. 314.-CONNECTION BETWEEN PRIVATE-BRANCH AND CITY SUB- 
SCRIBERS WHEREIN THE PRIVATE-BRANCH SUBSCRIBER CON- 
TROLS THE SUPERVISORY SIGNAL AT THE MAIN OFFICE, 

27 417 



418 



AMERICAN TELEPHONE PRACTICE. 



in series with a two-microfarad condenser between the two sides of 
the trunk line. Both sides of the signal circuit are broken at the jack 
when the plug, is inserted. 

The cord circuit uses two batteries, feeding respectively the 
answering and calling cords, through the double-wound magnets of 
the supervisory signals, A and B. These coils are not differentially 
wound, and the signals are therefore displayed as long as current is 
traversing them. In addition to the supervisory signals and the 
usual ringing and listening keys, each cord circuit is provided with 
two keys, K and K', each serving, when operated, to cut off battery 
from its cord and at the same time to close together the two coils of 
the supervisory signal so that they form a bridge across the cord 
circuit. Battery may, therefore, be cut off from that plug which is 
inserted into the trunk jack. 




FIG. 315.-CORD CIRCUIT FOR PRIVATE-BRANCH EXCHANGE SYSTEM. 



When two private branch lines such as are shown at the left 
of Fig. 313 are connected by the private branch cord circuit, the 
keys, K and K' are left in their normal positions. Current is then 
fed to each subscriber for talking through the two coils of the grid- 
iron signals, A and B, which signals are displayed as long as the 
subscribers are talking, but are effaced as soon as they hang up. If, 
however, the connection is between a trunk line and a main office 
subscriber's line, as shown in the figure, that cord which is inserted 
into the jack of the trunk line has its battery cut off by the key K 
or K', in which case the corresponding supervisory signal is ener- 
gized by current flowing from the main office as long as the con- 
nection is complete at both ends of the trunk line. It is evident that 
the presence of the signal A or B bridged across the trunk line at 
the private branch end will keep the supervisory lamp at the main 



PRIVATE BRANCH EXCHANGE SERVICE. 419 

office unlighted as long as the trunk line connection exists. This 
condition cannot be effected by any action of the private branch sub- 
scriber, and this lamp is only illuminated by the withdrawal of the 
plug from the trunk jack at the private branch board. When the 
private branch subscriber hangs up his receiver the signal will be 
conveyed to the private branch operator only, by operation of the 
supervisory signal A or B, connected with the cord making the 
connection with the subscriber's line. Upon seeing this signal the 
private branch attendant will pull down the connection, lighting 
one of the supervisory lamps of the cord circuit at the main board. 
When the main office subscriber hangs up his receiver, the main 
office operator will see both signals lighted and pull down the con- 
nection. 

An example of the first method of handling supervisory signals, 
that is, where the private branch subscriber during a connection 
controls a supervisory signal at the main office only, and where the 
disconnect signal at the private branch exchange is given by the 
withdrawal of the main office plug, is illustrated in Fig. 314. This 
shows the private branch subscriber's line circuit at the left, its 
method of operation being obvious. It also shows the trunk circuit 
extending from the private branch switch-board at the left of the 
figure to the main office switch-board at the right. 

Fig. 315 shows the cord circuit, which, in this case, is used only 
to connect two local lines. The operation of the cord circuit for two 
local lines will be obvious when it is stated that the third contact of 
the plug is adapted to control the normal circuit of the supervisory 
lamp, this circuit being completed through the grounded ring con- 
tact of the line jack into which the plug is inserted. When so com- 
pleted the control of the supervisory signals depends only on the 
energization of the supervisory relays A or B, through the coils 
of which current is fed to the subscribers' lines for talking pur- 
poses. So far, therefore, as the cord circuit and local lines are con- 
cerned, the operation is practically that of any common battery ex- 
change and the signals are conveyed in the same manner. 

Considering now the trunk circuit, Fig. 314, it will be seen that 
the trunk circuit terminates at the private branch exchange end in 
a plug and cord, and in fact resembles somewhat in its equip- 
ment the incoming end of the trunk circuits used in regular trunk- 
in-g between large exchanges. The equipment of this trunk circuit 
may be best described in connection with the description of its 
operation. 



420 



AMERICAN TELEPHONE PRACTICE. 



Assuming that a private branch subscriber calls for a main ex- 
change subscriber, the operator at the private exchange will, after 
answering the call with an answering plug of a regular cord cir- 
cuit, withdraw this plug and insert in its place the plug of a trunk 
line. 

The relay, A, is polarized and is kept normally in the position 
shown by current flowing from the main exchange battery. As soon 
as the trunk plug is inserted into the jack of the private branch 
line, the line relay at the main exchange, which had previously re- 
mained de-energized, due to the high resistance of relay A, will oper- 




FIG. 316.— PRIVATE-BRANCH EXCHANGE SWITCH-BOARD. 



ate, and thus cause the line lamp at the main exchange to be dis- 
played. The main exchange operator will answer in the ordinary 
way, and after finding out the number of the line with which con- 
nection is to be established, establish this connection in the ordinary 
manner. 

When the private branch subscriber hangs up, the answering 
supervisory signal at the main office will be lighted and the ringing 
lamp at the private branch exchange will also be lighted, due to the 
falling back of the armature of the relay C. To this the private 
branch attendant pays no attention. When, however, the main 



PRIVATE BRANCH EXCHANGE SERVICE. 421 

office operator, in response to the signals, withdraws the answering 
plug from the private branch trunk line jack, the relay A, at the 
private branch end of the trunk, will be restored to its normal posi- 
tion and allow its armature to fall back, thus lighting the disconnect 
lamp, which is now included in the circuit which may be traced 
from the live side of battery to the front contact of the relay B, 
thence through the lamp and back contact of the relay A to ground 
through the back contact of the relay C. 

With these circuits the private branch operator calls the main 
office operator by short-circuiting the two sides of the trunk line, 




FIG. 317.— PRIVATE-BRANCH EXCHANGE SWITCH-BOARD. 

which may be done by merely throwing her telephone in circuit by 
means of her listening key. It may also be done by plugging in to the 
short-circuited jack. 

In Fig. 316 is shown a type of private branch exchange board 
which is representative of good practice. By providing a desk at 
which the attendant may sit, she is thus enabled to attend to other 
work than that of operating the exchange, particularly in cases 
where there is not enough telephone work to keep her busy. This 
type of board is manufactured by the Kellogg Switchboard and Sup- 
ply Company, and in Fig. 317 is shown another view which gives 
some idea of how the apparatus is arranged on the connecting rack. 



422 AMERICAN TELEPHONE PRACTICE. 

This connecting rack is contained within the left-hand lower por- 
tion of the cabinet, which is adapted to be opened so as to expose 
most of the connections to view. In the case of large private branch 
installations the use of the multiple board often becomes necessary, 




FIG. 318.— MULTIPLE PRIVATE-BRANCH EXCHANGE BOARD. 



and where this is true the standard form of cabinet used in multiple 
board work will apply. 

In Fig. 318 is shown a multiple board installed by the Western 
Electric Company for the Chicago Telephone Company, this board 
being that of the private branch exchange of Marshall Field & 
Company in their large department store, this being the largest 
private branch exchange in the world. 



CHAPTER XXIV. 

PARTY LINE SYSTEMS. 

Probably no branch of telephone work has offered more induce- 
ments to the inventor and designer, and consequently received a 
greater share of ingenious application, than the party line problem. 

The term "party line/' or multi-party line, as it might be more 
properly called, refers to a line having more than two stations upon 
it. In this definition one of the stations may be a central office 
at which the line terminates. 

The term "party line" in exchange working is used in contradis- 
tinction to the term "individual line." An individual line serves one 
subscriber only, while a party line serves to connect two or more 
subscribers with the central office. 

Where it does not run to a central office a line connecting two tele- 
phones only would be called a "private line," but if the line con- 
nected more than two stations it would be referred to as a "party 
line." 

Party lines may be divided into two general classes: 

(i) Those on which the bells of all the telephones ring when 
any party on that line is called. In such lines a code of audible sig- 
nals is usually employed to enable the parties at the various stations 
to distinguish their calls from those of others. 

(2) Those where a system of selective signaling is employed, so 
that any one party may be called without ringing the bells, or in 
any way disturbing any of the other stations. 

The first of these classes may, for the want of a better name, be 
called "non-selective" party lines, and such lines may be divided 
into two general sub-classes, according to the connection of the in- 
struments on the line, as follows: 

(a) Series party lines where the signaling devices are connected 
in series in the circuit of the line. 

(b) Bridging party lines, on which the signaling devices are con- 
nected across the circuit of the line in multiple. 

The second of these general classes may be called "selective partv 
lines," and these may be divided into three sub-classes, as follows: 

(a) Those employing step by step movements to complete the 
calling circuit of the desired station. 

423 



424 



AMERICAN TELEPHONE PRACTICE. 



(b) Those employing currents of different directions or polarity 
for operating the different signals. 

(c) Those using the harmonic system of selecting — that is, where- 
in currents of various frequencies are used for actuating the differ- 
ent signals. 

Non-selective systems will be first considered. 
Probably the first party line ever constructed connected the in- 
strument in the line circuit in series, hence the name, "series party 




FIG. m— SERIES METALLIC PARTY-LINE CIRCUIT. 

line." Such a line connecting 4 instruments on a grounded circuit 
is shown in Fig. 319. In Fig. 320 are shown 6 instruments, con- 
nected in series in a metallic circuit. The usual form of instrument 
used in this class of work is the ordinary series magneto telephone, 
the circuits of which are shown in Chapter IX. 

A little consideration will show that one of the chief disadvantages 
of the series line is that the talking circuit of anv two stations en- 



IX 



IX 



QQ 



Y 



QQ 



^ 



QQ 



f'f< 



A-K 



2Q 






©^ 



FIG. m— SERIES METALLIC PARTY-LINE CIRCUIT. 

gaged in conversation must always pass through the bell magnets 
of all the other stations which are not in use. As these magnets 
necessarily possess a considerable amount of impedance, this is a 
very serious objection, and when a great number of instruments are 
used the speech transmission becomes very faint. For this reason 
it is customary to wind the bell magnets on instruments to be used 
on series lines to a low resistance, rather lower, in fact, than on 
the ordinary instrument for exchange purposes. Eighty ohms for 



PARTY LINE SYSTEMS. 



425 



each complete double ringer magnet has generally been the ap- 
proximate standard, the winding being of No. 31 B. & S. gauge 
single silk, insulated copper wire. A better arrangement, however, 
under most circustances, is to use coils wound as low as 40 ohms, 
or another good arrangement is to connect the two bobbins of the 
ordinary 80-ohm ringer in multiple instead of in series, thus giving 
a combined resistance of approximately 20 ohms. 

The armature coil on the magneto-generator in series party lines 




FIG. 321.— BRIDGING GROUNDED PARTY-LINE CIRCUIT. 

is, of course, not included in the talking circuit where modern 
instruments are used, on account of the fact that every generator 
is provided with an automatic shunt, which, as has already been 
shown, affords a path of practically no resistance about the gener- 
ator armature at all times except when the generator is in actual 
use. 

The number of bells that can be rung on a series party line is 
far in excess of the number that can be talked through. Thus, 50 
instruments in series may be satisfactorily operated, so far as ring- 




FIG. 322.-BRIDGING METALLIC PARTY-LINE CIRCUIT. 



ing is concerned, but even half that number would give intolerable 
talking service. 

Another objection to the series party line is that even where me- 
tallic circuits are used it is practically impossible to properly balance 
the line with regard to inductive disturbances. If all the instru- 
ments are placed in one side of the line, using the other side merely 
as a return, the line will be badly unbalanced on account of one 
of its sides possessing great impedance, while the other has onlv 



426 



AMERICAN TELEPHONE PRACTICE. 



that of the line wire. This may be partially remedied by alternately 
placing the instruments on one side of the line or the other, as is 
shown in Fig. 320, but often where this precaution is taken a per- 
fectly quiet line cannot be secured where the conditions are at all 
adverse. 

As a matter of fact, the series party line has no place in modern 
telephone practice, it having been almost universally superseded by 




FIG. 323.— CARTY BRIDGING BELL PARTY-LIXE SYSTEM. 



the bridging or multiple system of party line working, as disclosed 
in the now famous Carty patent.* 

In Fig. 321 is shown the method of connecting four instruments 
in accordance with the bridging system on a grounded line, and 
in Fig. 322 four instruments are similarly shown connected on a 
metallic circuit. It will be seen that the line wire in Fig. 321 or the 
line wires in Fig. 322 are continuous through all the stations, each 
instrument being placed in a separate bridge wire either between 
the line wire and ground or between the two line wires. 



* U. S. patent to John J. Carty, No. 449,106. 



. PARTY LINE SYSTEMS. 427 

The circuits of bridging instruments have already been discussed 
in Chapter IX. The line connections of an n-station metallic cir- 
cuit bridged line is shown in Fig. 323, this being a copy of the main 
figure in the Carty patent referred to. The various instruments, 2, 
3, 4, 5, etc., are connected across the two sides, / and l 2 , of the line, 
L. Some of the instruments are represented merely by circles, but 
at station 9 the complete circuits are shown in detail. 

If a station is not located directly on the route of the line it is con- 
nected simply by running two lateral wires, 13 and 14, from its bind- 
ing post to the most convenient point on the through route of the 
line. 

In this system the call-bells, P, at each station are permanently 
bridged across the two sides of the line, and are made of high 
resistance and retardation. The generator, G, at each station is 
in a separate bridge circuit, which is normally open, but closed 
when the generator is operated. The talking circuit of each 
instrument, containing the receiver, R, and secondary winding of 
the induction coil, I, forms a third bridge circuit, which, like the 
generator circuit, is normally open. 

The telephone circuit of each instrument is automatically closed 
when the receiver is removed from its hook for use, and this 
operation also closes the local circuit containing the primary of 
the induction coil, /, the local battery, b> and the transmitter, T. 
In order that there shall not be an undue leakage of the voice 
currents through the permanently bridged call-bell circuits, the 
magnets of these call-bells are wound to a high resistance (usually 
a thousand ohms) and are also constructed in such manner that 
they will have a high coefficient of self-induction. When a 
generator at any one station is operated, it is connected across 
the two sides of the line in parallel with all of the call-bell mag- 
nets on the line. Part of the currents in this generator will, there- 
fore, pass through each of the call-bell magnets on the line, thus 
causing them all to operate if the amount of the current generated 
is sufficient to accomplish this result. The successful operation of 
this system depends on the fact that a coil possessing a high co- 
efficient of self-induction will transmit with comparative ease alter- 
nating or pulsating currents of low frequency, while it will form 
a practical barrier to similar currents having a very high frequency. 
The currents generated by the calling generator at any station are 
of sufficiently low frequency to pass with comparative ease through 
the call-bell magnets arranged along the line, while the rapidly al- 



428 AMERICAN TELEPHONE PRACTICE. 

ternating voice currents impressed upon the line by the telephonic 
apparatus at any station will be compelled to pass over the main 
line to the receiving station without being materially weakened by 
leakage through the call-bell magnets. At the receiving station 
these voice currents will pass through the telephone receiver and 
secondary coil of the induction coil, these being connected across 
the line at that station by virtue of the receiver being off its hook. 
This path through the receiving instrument is of comparatively 
low resistance and retardation, and thus practically takes all of 
the current from the distant station. 

The high retardation of the ringer magnets is obtained by 
winding them to a high resistance with a comparatively coarse 
wire so as to obtain a large number of turns in the winding. 
The length of the cores is increased for the double purpose of 
getting more iron in the magnetic circuit, and therefore a higher 
retardation, and also for affording a greater amount of room for 
the winding. The Western Electric Company wind their coils 
to a resistance of iooo ohms per pair, using No. 33 single silk mag- 
net wire. Many other companies use No. 38 wire and wind to a re- 
sistance of 1200 or 1600 ohms per pair. This does not give such good 
results, however, as using the coarser wire and the lower resistance 
and long cores. Some companies wind, or once wound, their bridging 
bell magnets partly with German-silver wire in order to make a high 
resistance at a low cost. They should learn, however, that resist- 
ance in itself is not the thing desired, but a great number of turns 
in the winding, which, of course, incidentally produces a high 
resistance. 

The generators for bridging systems should be designed for quan- 
tity rather than for high pressure, since they have to supply current 
to pieces of apparatus arranged in multiple. The fact, however, 
that a certain amount of voltage is also needed must not be over- 
looked, for, on long lines, the actual resistance of the line wire is 
often great, and unless the voltage is sufficient the bells at the dis- 
tant end of the line will not be rung, no matter how great the cur- 
rent output of the generator may be. On long lines, heavily 
loaded, sufficient current must be generated to ring the bells in 
multiple, and sufficient voltage maintained to ring the bells at the 
farthest end of the line. The question is, therefore, one of watts 
or horse-power, it being necessary for the generator to hold up 
this voltage to the required amount to ring the farthest bell on the 
line even when delivering its maximum amount of current. In 



PARTY LINE SYSTEMS. 429 

order to secure this greater amount of energy it is customary to use 
a stronger magnetic field, and this is attained by using four or five 
permanent magnets in the field instead of three, as is ordinarily prac- 
ticed in series generators. The winding of the generator is also 
of much importance and must vary to meet different requirements. 
A generator wound to 350 ohms with No. 33 single silk-insulated 
wire is perhaps best adapted to meet the average requirements of 
bridging party line service. 

All things considered, it is probably better to use low wound in- 
duction coils on bridging lines, so that the voice currents coming 
along a line will find a readier path through the talking circuit of 
the station receiving than through the call-bell bridges at the vari- 
ous stations. In many cases 500 and even 1000-ohm induction 
coils have been used on bridging circuits, which render the im- 
pedance of the talking circuit high, exactly what should be avoided 
when talking efficiency is considered. 

High impedance and resistance in the talking circuit is, how- 
ever, of advantage in cases where the receiver at some station has 
been accidentally left off its hook, as, in this case, a high resistance 
coil does not afford such a low shunt to the signaling currents, and 
therefore allows the bells at the other stations to be more readily 
rung than if the low resistance coil had been used. 

On country lines employing many stations, where the telephone 
forms the chief means of social as well as business intercourse, it 
is customary for a great many of the parties on a line to take down 
their receivers whenever their bells ring, no matter whether the 
call is for their station or not. This is a practice which it seems 
impossible for the managers of telephone lines to prevent, and fre- 
quently the result is such that the line is thus subjected to such a 
material shunt as not only to cut down the talking efficiency, but to 
make it impossible to ring. In order to render it possible to ring 
over such a line, regardless of whether the receivers are down or 
up, a scheme has been proposed, and put into practice to some 
extent, of connecting a one-microfarad condenser or a non-inductive 
resistance in circuit with the secondary of the induction coil and the 
receiver at each station. This condenser does not materially cut 
down the talking efficiency in receiving, while it does very materially 
increase the effective resistance of the shunt with respect to the ring- 
ing curren f s. In other words, the condenser affords practically no 
barrier to the high frequency talking currents, while it does ma- 



430 AMERICAN TELEPHONE PRACTICE. 

terially cut down the transmission through it of the comparatively 
low frequency ringing currents. 

In connecting a party line with a switch-board much trouble 
is often caused by the use of an improperly wound annunciator 
coil. It should be borne in mind that the drop magnet really 
bears the same relation to the line as the ringer magnets, in the 
various telephones, and should therefore be connected in the 
same way. For a series party line the switch-board drop should 
be wound to about the same resistance as the ringer magnets. If 
the resistance is made higher, as is often done in the attempt to 
secure a more sensitive drop, two parties on the same line will have 
much difficulty in talking to each other, because the drop is in 
series in the line. 

In the bridging-bell system the impedance of the switch-board 
drop should also be about the same as that of the ringer mag- 
nets. It is frequently impossible to wind drops to iooo ohms on 
account of insufficient wire space, and a tubular drop wound to 
500 ohms is perhaps most common. A properly designed bridged 
drop may be left permanently bridged across the line, to serve 
as a clearing-out drop when the subscribers are through talking. 
In small exchanges, operating party lines, it is customary for the 
operator at such a switch-board to distinguish between the calls 
for a connection with some other line, and those which are for 
parties on the same line, by means of the buzz caused by the 
vibration of the armature of the drop. It is, therefore, desirable 
to give the drop armature a rather wide adjustment, so that it 
will make enough noise to enable the operator to readily dis- 
tinguish the signals. The combined ringer and drop, such as is 
shown in Fig. 198, gives better results in such a case, if there is 
room enough for it on the face of the switch-board. 

On lines where a toll rate is charged, much loss of revenue is 
often caused by surreptitious conversations, that is, by parties on 
the same line calling each other and carrying on their conversation 
without the knowledge of the switch-board operator, so that no 
means is afforded for properly charging the use of the line against 
them. Many arrangements of circuits and apparatus have been 
devised for obviating this difficulty. One of these, which is suitable 
only for bridging lines, is to provide at the central office a switch- 
board drop of extremely low resistance and so arrange it that it 
will be cut out upon the insertion of the plug. The low-resistance 
path through this drop acts practically as a short-circuit to all of 



PARTY LINE SYSTEMS. 431 

the high resistance bells on the line, so that when a party rings, 
nearly all of the current from his generator passes through the 
switch-board drop, without actuating any of the bells. When the 
operator plugs in for conversation, or for the purpose of calling 
up some subscriber on that line, the low-resistance drop is cut 
out, so that the line is no longer short-circuited. This method 
cannot be used on any except short lines, because the resistance 
of the drop, in addition to that of the line wire, proves high enough 
to shunt some of the current through the magnets of the bells at 
the distant end of the line, when parties at that end attempt to signal 
each other. While the drop would short-circuit the end of the line 
nearest the switch-board, the instruments at the farther end would 
not be appreciably affected, owing to the high resistance of the line 
wire between them and the board. 

This method is not, on the whole, very satisfactory, and a better 
one is to arrange the magnetos at the subscribers' stations to gen- 
erate a current in one direction only, instead of the usual alternating 
current, and to use biased ringers at all the stations, adapted to ring 
only on pulsations of a different polarity from those of the gen- 
erators. The switch-board drop, also bridged across the line, is 
of a non-polarized type, so as to fall when actuated by currents in 
either direction. Thus, when any subscriber calls, the current will 
have no effect upon any of the ringer magnets of the other sub- 
scribers, because it tends only to pull the armatures closer to the 
poles toward which they are already attracted, but will cause the 
switch-board drop to fall in the ordinary manner. Thus, no sub- 
scriber can obtain a conversation with any other subscriber without 
the full knowledge of the operator. The switch-board generator 
is equipped for sending out currents, of the opposite polarity from 
those generated by the subscribers' generators so that the operator 
may ring up the subscribers at will. 

Coming now to selective signaling on party lines, those systems 
employing "step by step" mechanisms will first be considered. In 
these the general scheme is to employ a disc at each sub-station, each 
capable of being revolved one step at a time by pawl and ratchet 
mechanisms controlled by electromagnets. The discs at all the sub- 
stations on a line may thus be revolved in unison by currents sent 
out from the central office. At some point in the rotation of each 
disc certain electrical contacts are made or broken in such a way 
as to render the talking and call-receiving apparatus at the corre- 
sponding station operative. The amount of rotative movement thus 



432 AMERICAN TELEPHONE PRACTICE. 

required to complete the circuits differs with respect to the discs at 
the various sub-stations on the line, and therefore the operator 
at central may successively bring each station into such condition 
as to allow the use of its talking and call-receiving apparatus. The 
operator may thus pick out the desired sub-station and ring its bell 
to the exclusion of all others on the line. 

The use of step by step mechanism in this class of telephone 
work has apparently from the very first offered the most plausible 
solution of the problem of selective signaling, and while there are 
no insurmountable obstacles in the way of its being put into success- 
ful practice, yet the fact remains that there are comparatively few 
lines being operated on this plan ; and it is only recently that one of 
the large manufacturing companies has brought the system into 
what might be called a fit condition for extensive commercial use. 

One of the very first to apply step by step mechanism to the party 
line problem was E. N. Dickerson, Jr., as early as January, 1879. 
In this two revolvable discs were mounted on a common shaft at 
each sub-station. One of these was of insulating material, but had 
at one point on its circumferential surface a conducting segment. 
The other was of the reverse construction, having nearly all its sur- 
face of conducting material. The first-mentioned disc controlled 
the call bell circuit at each station, the bell being placed under the 
control of the operator when the conducting segment on the disc had 
been stepped into such position as to make contact with wiping 
springs resting against the cylindrical surface of the disc. At this 
point in the rotation of the bell controlling disc the other disc, which 
controlled the talking apparatus, had its insulating segment oppo- 
site the springs bearing against the surface of this disc, and there- 
fore a short circuit was removed from the receiver. 

All discs could be released at the end of a conversation by throw- 
ing a strong current on the line, which, by operating another set 
of magnets caused all the pawls to be lifted from the ratchet wheels 
connected with the discs, thus allowing the discs to be returned by 
springs to their normal positions. 

The conducting and insulating segments on the rotary discs occu- 
pied different relative positions at the different stations, the ar- 
rangement being such, for instance, that the segments occupied 
such a position on the discs at the first station as to be brought in 
contact with their contact springs upon the first step of their move- 
ment. As the segments at the other stations all occupied different 
relations it follows that the first step places the first station in posi- 



PARTY LINE SYSTEMS. 433 

tion to be called' while all of the other stations could only be reached 
by making succeeding steps, as, for instance, two steps for the 
second station, three for the third and so on. If, therefore, it was 
desired to call the third station on the line three impulses would be 
sent on the line, after which that station would be called. 

At almost the same time George L. Anders produced a step by 
step system dependent on a somewhat different idea. All of the 
call bells were left permanently on the line wire and their hammers 
were actuated in unison when a pulsating current was sent over the 
line. A notched disc at each station prevented the bell hammer at 
its station from striking the bell except at such time as the notch 
on the disc was opposite the rod which carried the hammer. The 
discs were so arranged as to be stepped around by vibrations of the 
bell hammer, while the impulses of one polarity were sent over the 
line. In calling a certain party a sufficient number of impulses in 
this direction were sent to bring the notch of the disc at the desired 
station into a position opposite the bell-hammer rod, after which 
currents of the opposite polarity were sent over the line to ring that 
bell. Currents of this direction did not actuate the stepping de- 
vice, but did actuate all of the bell hammers as before, but the notch 
in the disc of the desired station allowed that bell to sound, while 
the notches at the other stations, not being in the proper position 
prevented their bell hammers from striking the gongs. 

Dickerson used his stepping device to control the local circuits 
at each station. Anders left all of his circuits unaltered and used 
his stepping device to control merely the length of the stroke of the 
bell hammer. 

These two systems, produced in the very early days of the art of 
telephony, have been followed by a great number of others, some 
of them worked out with more completeness and embodying features 
of more or less merit. It is surprising, therefore, that with the need 
of a selective party line system clearly in the minds of telephone men, 
and after the necessary fundamental ideas had been developed, 
almost at the very beginning of the telephonic art, commercially 
practical systems have not been produced and found general favor ; 
especially when the marvelous effectiveness of step by step move- 
ments, when applied to the telegraphic art, as in the various ticker 
systems, are considered. 

The Stromberg-Carlson Telephone Manufacturing Company 
is now putting on the market in large quantities a step bv step sys- 
tem for party line signaling, which appears to be meeting with great 

2S 



434 AMERICAN TELEPHONE PRACTICE. 

favor, particularly on country lines, and from all appearances is a 
practical success. This is adaptable to lines having as many as 
twenty stations. 

Coming now to the more commonly used method of operating 
party line signals selectively, employing currents in different direc- 
tions, or of different polarities, for operating the different signals, 
we find that this plan was well known in telegraphy, even before the 
birth of telephony. 

The duplex and the quadruplex systems of telegraphy of the pres- 
ent time offered the best possible demonstration of the utility and 
practicability of systems operating on this principle when properly 
developed. The quadruplex system not only admits of sending se- 
lective signals one at a time, but even allows four to be transmitted 
simultaneously over a single grounded circuit, two in one direction 
and two in the other, a result never even seriously attempted in 
telephonic signaling. It may be said that at present by far the great- 
est number of successfully operating party line systems are based on 
this plan. 

As in the case of the step by step system, George L. Anders was a 
pioneer in this line also, and in 1879 he produced a two-party line 
system having the call bells at the two stations polarized oppositely 
and included directly in series in the line wire. This was the birth 
of what is called the "biased bell," the operation of which was de- 
scribed in connection with Fig. 104 in the chapter on Magnet Call- 
ing Apparatus. With two such bells included in series current in 
a positive direction would therefore operate the bell at one station, 
and current in a negative direction that at the other. 

Abandoning Anders' series system, using two bells in series in the 
line wire, and adopting the bridging system instead, and applying 
the ideas to a metallic circuit line instead of a single grounded line, 
Mr. Angus S. Hibbard, of the Chicago Telephone Company, has 
produced a four-party line wherein any one of the four stations may 
be called from the central office without disturbing the other. This 
is the system now almost universally used by the Bell companies, 
it being, however, modified to some extent to meet the various re- 
quirements of service. 

The principle of the Hibbard system, so far as the ability to ring 
any subscriber on the line is concerned, is illustrated in Fig. 324, 
where the two limbs, a and b, of the metallic circuit line are shown 
extending from four subscribers' stations to the central office. The 
bell of station 1 is legged from the limb, b, to ground and connected 



PARTY LINE SYSTEMS. 



435 



in such direction that only a negative current flowing from the line to 
ground will operate it. The bell at station 2 is similarly connected, 
except that its polarity is reversed so that it will be operated only by 
positive currents. In similar manner the bells at stations 3 and 4 are 
connected with respect to the limb, a, of the line. 

At the central office is connected a pulsating current generator 
adapted to send out current in a positive or negative direction, ac- 
cording to which of its terminals is connected with the line. If, 
therefore, the positive pole of the generator were connected by 
means of the plug, c, and the switch, d, to the terminal of the limb, a, 
station No. 4 would be rung. If the switch, d, were reversed thus 
connecting the positive pole to ground, and the negative pole of the 
generator to line, the bell at station 3 would be rung. In the same 
manner, by applying the plug, e, to the terminal of the limb, b, either 




FIG. 324.-HIBBARD PARTY LINE IN THEORY. 



station 1 or 2 would be rung. The actual circuits of such a system 
adapted to common battery talking and signaling is shown in Fig. 

325- 

This, as will be seen, employs four ringing keys, 1, 2, 3 and 4, in 
the cord circuit, the operation of any one of which will cause the 
bell of the correspondingly numbered station on the line with which 
the calling plug is connected, to ring. Thus, depressing key, I, will 
throw positive pulsations from the generator on the sleeve side, a, 
of the line, at the same time grounding the tip side, a! . This will 
cause the bell at station 1 to ring. Similarly any of the other sta- 
tions may be called up. 

In order to prevent undue complication of the ringing keys, and to 
bring about the further advantage of having a party line handled, so 
far as the operator is concerned, in exactly the same manner as if 
it were an individual line, Mr. F. R. McBerty, of the Western Elec- 
tric Company, has proposed a very ingenious system, shown in Fig. 
326. This uses a separate jack on the switch-board for every sub- 




FIG. 825.-HIBBARD PARTY LINE SYSTEM. 
436 



PARTY LINE SYSTEMS. 



437 



scriber on the party line ; thus, on a four-party line four jacks would 
be used for each line, instead of one, as in the original Hibbard sys- 
tem. 

The arrangement of the subscriber's apparatus with respect to the 
line is the same in all respects as in the system just described, and 
the operation of calling central is obvious from the diagram. 

In connection with the line conductors, a and a' are four spring- 




FIG. 326.-McBERTY'S MODIFICATION OF HIBBARD SYSTEM. 



jacks, i, 2, 3 and 4. Each of these has a short line spring, ft, a 
long spring, n', and a tubular thimble, n 2 . The connection of these 
springs and thimbles to the conductors of the line is different in the 
case of each jack as examination will readily show. 

The switch-board is provided with the usual plugs and cord cir- 
cuit, which includes a ringing key. This key, in addition to the 
usual pair of switch springs and their normal and alternate contact 



438 AMERICAN TELEPHONE PRACTICE. 

anvils, has a spring, p, which is adapted to register with an anvil, />', 
when the key is operated. 

The arrangements of the jacks with respect to the line wires are 
such that the mere insertion of the calling plug in any jack will 
establish the proper relations between the generator and the line, 
to operate the bell at the corresponding station upon the depression 
of the key. Thus, suppose the operator wishes to call station i. She 
inserts the plug in jack, i, of that line, and depresses the ringing key. 
A pulsatory current in a positive direction will now flow from the 
tip of the plug to the spring, n, of the jack and to line conductor, a, 
and thence through the bell at station i to ground. The limb, a', of 
the line will be grounded through the jack spring, n' , and ring, n 2 , 
and thence through the spring, p, of the ringing key. The bell will 
be operated by this current. The bell at station 2 will also receive 
part of this current, but not be operated on account of its polarity. 

By tracing out the circuits through the other jacks it will be 
found that in each case the spring-jack into which the plug is in- 
serted determines w r hich of the signals connected with that line 
shall be operated. 

When the operator has made a connection with any spring-jack, 
and has operated the signal at the corresponding station, the pres- 
ence of the plug in that spring- jack indicates to her, during the 
existence of the connection, the station which has been signaled. 
If it should be necessary to signal the same station again, she does 
not have to remember w r hich party on that line has been signaled, for 
she may be sure of again calling the same one by merely pressing 
the key. If it should be necessary to make any charge, as in the 
case of a toll connection, the identity of the station signaled is ascer- 
tained by the presence of the connecting plug in the corresponding 
spring- jack. 

The arrangement of polarities, circuits and symbols in Fig. 326 
differs from that of preceding figures. It is obvious that any arbi- 
trary arrangement of polarities, line conductors and station numbers 
may be adopted so long as any given telephone exchange is con- 
sistent throughout. 

A considerable amount of difficulty has been experienced in adopt- 
ing the Hibbard party line system to operate properly in conjunction 
with standard common battery circuits as employed by either the 
Bell or Independent companies, the principal difficulty occurring in 
securing the proper operation of the line and supervisory signals. It 
has been shown that nearly all common battery systems depend for 



PARTY LINE SYSTEMS. 439 

signaling on the fact that the line circuit is open at the subscribers' 
stations when the receivers are on their hooks, and is closed when the 
receivers are removed for talking. The normally open condition of 
the line is attained by the use of condensers in series with the bells, 
the condensers serving to allow the passage of the alternating cur- 
rents used in ringing, but to prevent the .passage of direct current 
due to the potential constantly placed on the line by virtue of its 
connection with the battery at the central office. Unfortunately, 
however, where "biased" bells are used, the condenser must be 
abandoned, because the effectiveness of biased bells demands the use 
of pulsating currents, or currents in one direction only. When such 
a current is applied to a condenser the result is naturally an alter- 
nating-current, and therefore biased bells would ring on either 
polarity of current if placed in series with condensers. 

Since in the Hibbard system two bells in multiple are connected 
between each side of the line and ground, it follows that the two sides 
of the line are connected through a combination of bells, two in 
multiple and two in series, giving, therefore, an effective resistance 
of one bell only. If the bells were of ordinary resistance, therefore, 
this arrangement would keep the line signal operated at all times, 
and would also operate the line, or the supervisory relay, whether the 
receiver was on or off the hook. In order to remedy this the bells 
are usually wound to a very high resistance, usually 2500 ohms, and 
placed in series with a non-inductive resistance in some cases as high 
as 20,000 ohms. Even with this resistance the work of the relays is 
marginal, that is, the relay must be so adjusted as to work properly 
through the resistance of the line and the talking apparatus, but it 
must be made so insensitive as not to work through the resistance of 
the line and that of the bells. Such conditions are to be avoided, if 
possible. 

A modification which is being used to a considerable extent by the 
Bell companies, and which effectually removes the difficulty due to 
the marginal adjustment of the line and supervisory relays, has been 
devised by Messrs. Thompson and Robes. In this system each station 
is provided with a relay especially designed for use with alternating 
currents, each relay being bridged across the metallic circuit of the 
line in series with a two-microfarad condenser. This arrangement 
is shown in Fig. 327, which shows two four-party lines connected 
by the usual Bell multiple switch-board circuits at the central office. 
The contacts of each of the relays at the subscribers' stations are so 
arranged that when the relay is operated it will connect the usual 




FIG. 327.— THOMPSON & ROBES PARTY-LINE SYSTEM. 

440 



PARTY LINE SYSTEMS. 441 

biased bell to earth from one leg or the other of the metallic line, 
such connection being made only during the process of ringing. 
The ground branches are detached from the line at all times, except 
during the ringing operation, but are made available for use in 
ringing by the action of ringing when pulsating current is thrown 
on the line. 

When the proper (say, negative) pulsating current is applied 
between the sleeve side of the line and ground, current will flow out 
to the sleeve side of the line through the condensers at all of the 
stations in multiple, and back to the ground on the tip side of the 
line at the ringing key. All of the relays at the subscribers' stations 
will be thus operated, which will establish new ground connections 
at the subscribers' stations. The negative current then in passing 
through the bell at station No. I to ground will ring that bell, while 
the bell at station No. 2 will not be rung, because of the current being 
in the wrong direction. 

This solution has the advantage of removing the necessity for the 
marginal adjustment of the line and supervisory relays at the central 
office, but it has the disadvantage of placing an extra piece of appa- 
ratus, the relay, at each of the subscribers' stations. 

There is another system of selecting by polarity on party lines, 
known as the B-W-C system, from the names of its inventors, 
Messrs. Barrett, Whittemore & Craft. While this system is not now 
in use, so far as the writer is aware, it was at one time put into wide 
use by many of the Bell operating companies, and for that reason 
and because of the several exceedingly ingenious features in it, it 
is thought worthy of a careful description. It was not, however, a 
commercial success, on account of the trouble in its maintenance ; 
and in that respect it serves as an additional example of the fact 
already pointed out, that undue complexity at the subscribers' stations 
cannot be tolerated in commercial telephony. This system depends 
for its operation on the sending of currents of either polarity over 
either or both of two line wires in combination with each other or 
with the ground. Thus calling one wire A, and the other B, and 
representing the ground by G, it is evident that without using wire 
B at all, a current in either direction could be sent over wire A, 
with a ground return, thus giving means for two selective signals. 
Similarly leaving A out of the question, a current of either direction 
could be sent over B, with a ground return, thus providing for two 
other selective signals. So far the combinations are identical with 
those of Hibbard. A current may also be sent in either direction 



442 



AMERICAN TELEPHONE PRACTICE. 



over the metallic circuit formed by A and B, thus providing for two 
other signals; and lastly, by using A and B in multiple, currents 
could be sent in either direction, using a ground return, thus 
affording means for two more signals, or eight in all. Two other 
combinations might be obtained by sending currents in either 
direction over wire A, using wire B and the ground in 
multiple as a return; and similarly two others by using B for one 
side of the circuit with the wire A, and ground in multiple 





Line A. 


Line B. 


Ground. 


I 


-f 
O 

o 

+ 

+ 


O 

O 

+ 

+ 
+ 




2 


-K 


7. 




A 


+ 


C 


o 


6 


o 


7 




8 


+ 







for a return. These latter combinations, however, have been 
found to introduce undesirable features, as will be readily under- 
stood. The eight desirable current combinations may be tabu- 
lated as in the table above. 

In this table the plus and minus signs indicate which pole of 
the calling battery at central is connected to either line wire or 
ground. Thus, in the first combination, the positive pole is con- 
nected with line A, the negative with the ground in order to utilize 
the earth return. Line B in this combination is not used at all. 

Fig. 328 shows diagrammatically such an arrangement of appa- 
ratus at eight stations that the call-bell, D, at each station will be 
actuated only when the one particular set of current combina- 
tions is sent over the line. A and B represent two line wires ex- 
tending from a central station, C, to a number of sub-stations, 
S, S 2 , S B , etc. At each of the sub-stations are two relays, R and R 2 , 
placed in earth branches, m and q, from the two line wires, A and 
B, respectively. These two branches are united at e, and connected 
with the ground at G. The signal bell, D, is connected with the 
local battery, s, in a circuit, the continuity of which is controlled 
by each of the relays, R and R 2 . Unless the armatures, 13, of both 
relays rest against their back stops, 12, the local circuit containing 
the bell will be opened at one or two points. The relays of each 
station differ in some way, either in construction or arrangement, 



tfpraetf 










1 13 



444 AMERICAN TELEPHONE PRACTICE. 

from those of all other stations. Thus at station S the main con- 
ductor, A, is branched through a polarized relay made responsive 
to positive currents from the central office, and the main conductor, 

B, through a neutral relay, R 2 , adapted to respond to currents of 
either direction from the central office. It is thus obvious that if 
a positive current is sent over wire A without sending any current 
whatever over B, the bell at station 5 will be operated because 
the positive current will cause the relay, R, to release its armature, 
while the armature of relay R 2 is already released. Thus, both 
contacts, 10 and n, will be closed and the bell circuit complete. 
Station S 2 also has a neutral relay on wire B and a negatively 
polarized relay on wire A. The third and fourth stations, S 3 and 
S 4 , each have a neutral relay on wire A and a positively or nega- 
tively polarized relay on wire B. The fifth station, S 5 , has two 
polarized relays, one adapted to respond to positive currents and 
attached to wire A, and the other to negative currents and attached 
to wire B. The sixth station, S Q , also has oppositely polarized 
relays, but their connection with the line is the reverse of that in 
station S 5 . The seventh station, S 7 , has two positive relays and 
the eighth station, S 8 , two negative relays, one in each case being 
bridged between each limb of the line and ground. 

Reference to the table of current combinations will show, in 
connection with Fig. 328, that the sending of any particular 
combination to line will operate the relays of the station bearing 
the corresponding number in such manner as to close the local 
circuit at that station. Further consideration will also show that 
no combination will so operate the relays at more than one 
station. 

At the central station, B' is a generator of calling current, and 
C an earth connection complementary to the earth connections, 

C, at the sub-stations. K is a group of signaling keys, each cor- 
responding with one sub-station appliance, and when any partic- 
ular key is pressed it sends the proper current combination to 
line so that the relays at the particular sub-station represented by 
it will co-operate to close the local circuit and give the signal 
there ; but at the other stations no such effect will take place. 
Hence, to give a signal at any desired sub-station, it is only 
necessary to operate the particular key representing such station. 
To accomplish this, branch terminals are brought from the line 
conductors, A and B, from the ground connection, G', and from 
the positive and negative poles of the battery to the various ter- 



PARTY LINE SYSTEMS. 445 

minals on the signaling keys. The arrangement of the terminal 
contacts in each key is different, the differences corresponding with 
those of the sub-station relay arrangement. 

To illustrate: in key No. i the contacts are so disposed that its 
operation will connect conductor A, with the positive pole of the 
battery, £?', at contacts, v and y, the minus, pole of the generator 
with the earth terminal-contacts, z and w y and will leave conductor, 
B, disconnected. By this means a positive current is sent over 
line A, and is distributed through all the A relays at all of the 
sub-stations in parallel, rinding return through the earth branches; 
but as no current is transmitted over line conductor B, all of the 
eight B relays will remain unaffected. Under these conditions re- 
lay R, at station S will close point, 10, of its local circuit, and the 
point, ii, being already closed by the armature of relay R 2 , the nor- 
mal position of which has not been changed, the local circuit, c, of 
station S will be closed and the bell at this station will be rung. 
Station S 2 will not be signaled, because plus currents have no 
effect on its polarized relay, R. Station S 3 is not signaled, because 
the effect of the plus-current on line A is to attract the armature 
of neutral relay R, and thus open the local circuit, which is already 
open at point, n. Station S* receives no signal for the same 
reason. Station S 5 is not signaled, because, though the positively 
polarized relay on A closes the open point, 10, of its local circuit, 
the said circuit remains open at n, there being no current on B, 
station S Q because neither relay is acted upon, R being of minus 
polarity and R 2 having no current; station S 7 , because R alone is 
operated, and station vS 8 because both relays are of minus polarity. 

In applying the principles already pointed out to a practical mul- 
tiple-station circuit, it is desirable to reserve two of the current 
combinations for the operation of locking devices common to all 
stations. 

The seventh and eighth combinations in the foregoing table 
have been found most convenient for this purpose. The seventh, 
that is, the positive current over both conductors, A and B, in 
parallel, is used for locking the telephone apparatus at all sta- 
tions, and a negative current over both lines for unlocking the 
apparatus. Six combinations are thus left for signaling. 

The locking device and a visual busy signal are shown in asso- 
ciation with complete telephone equipments at two stations in 
Fig. 329. In these an additional electromagnetic apparatus, R 3 , 
is shown in circuit with the relavs, R and R 2 , at each sub-station, 



446 AMERICAN TELEPHONE PRACTICE. 

half of its winding being in the earth branch, in, of the relay R, 
and half in the branch, q, of the relay R 2 . 

Two electromagnetic helices, a and b, have the ends of their 
cores joined by soft-iron yoke-pieces to form the instrument, R 3 . 
Two soft-iron polar extensions, h and /, project inwardly from 
the yoke-pieces as shown. A polarized bar armature, /, pivoted at 
f", has one of its poles projecting between the pole-pieces, h and 
f, and adapted to move to one side or the other under the in- 
fluences of said pole-pieces. If current is passed through coil a, 
only, the magnetic polarity developed will be short-circuited 
through the yoke-pieces and the core of coil b, so that very little 
strength will be manifested in the pole-pieces, h and f ; if current 
be applied to the coil b only, the magnetic polarity will be simi- 
larly short-circuited, and, again, little effect will be manifested in 
the pole-pieces. Again, if current be applied to both coils, g, and 
b, so as to act in a complementary direction, the yoke-pieces will 
satisfy the magnetic flux with very little polarity in h and f; but 
if current be applied to coils, a and b, in inductiviely opposed di- 
rection, as will be the case when the seventh and eighth combi- 
nations are transmitted, consequent poles of full strength and 
opposite polarity will be formed at h and /. The polarized lever, 
j, is, therefore, actuated by the seventh and eighth current com- 
binations and remains unaffected by all others. 

As shown at the right of Fig. 329, the lever, /, serves not only 
as a lockout device, but also as a busy signal. The apparatus 
is shown in its locked or busy position at station S 2 of this figure 
and in its unlocked or free position in station S 3 . When the lower 
portion of the lever is moved to the left it forms a stop to lug, J 3 , on 
the hook-switch, L, and thus prevents the latter from rising should 
the receiver be removed from the hook. At the same time the 
small target, B, on the other end of the lever is displayed through 
a hole in the box, thus showing the party at that station that the 
line is busy. When in its other position the busy signal is not dis- 
played and the hook-switch is free to rise. 

When the operator at central presses the locking key, say key 
No. 7, all of the locking levers on the line, including that of the 
party to be called, will be actuated. In order that the party 
being called may not be thus locked out, the windings, 27 and 28, 
are provided around the polar extensions, h and /, on each in- 
strument. This winding has no function except at the station 
being called. In that station part of the current from the local 




i-A 



'.'-I ff 






,|j l > .!) ^ t | o" 51 d " * f"^ 




Ff^ ^B ' kra - a k ! 3 



to 




t § 



ffe 



s 



3 






^ 







*& *5 



^ 



s 



v*tt 



* a 



: H ^ 



!- •* «■! 



II 



ja 
5> 







417 



448 AMERICAN TELEPHONE PRACTICE. 

circuit, which is closed only at that station by the action of the 
relays, finds path through this winding, and the magnetism so 
developed serves to unlock the mechanism and to allow the party 
at that station to use his instrument. 

In Fig. 330 is shown a six-party line, the equipment at each 
station being of a similar character to that shown in Fig. 329, but 
simplified for the purpose of clearer illustration. The two sides 
of the line terminate in the line springs of a spring-jack, /, which 
springs normally rest on anvils connected to the windings, 31 and 
32, of a differentially wound switch-board drop. These two wind- 
ings pass around the core of the drop magnet in opposite direc- 
tions, after which they unite at the point, 60, and pass to ground 
through a battery, B 2 . The relative direction of the windings on 
the drop is such that the current from this battery circulates 
around the core in opposite directions, and thus does not affect 
the drop. It then divides equally between the two main con- 
ductors, A and B, and finally returns by the ground connections, 
G, at each of the several stations. The current thus flowing to 
the two conductors from the battery, B 2 , is in a negative direc- 
tion, and thus tends to maintain the apparatus at the several sta- 
tions in its unlocked condition. 

When any subscriber removes his receiver from the hook, the 
short arm of the hook-lever, L, makes contact momentarily with 
the spring, d, which grounds the main line wire, B, and thus allows 
a heavy current to pass through the winding, 32, of the drop, /. 
This throws the drop and attracts the attention of the operator. 
The operator answers the call in the ordinary way by the inser- 
tion of one of the plugs, P, with which the ringing keys, K, in Fig. 
328 are associated. 

When a sub-station is to be signaled, the calling plug, P, is in- 
serted into the spring-jack, which cuts off the annunciator and 
connects the keys, K, with that particular circuit. Key No. 7, which 
sends the plus current over both lines in parallel, is then oper- 
ated to lock the apparatus at all stations without ringing any of 
the bells; and then the key representing the desired station is 
pressed which results in ringing the bell, and at the same time 
in releasing the telephone apparatus at that station by the means 
already described. At the close of any conversation Key No. 8, 
sending a minus current over both lines in parallel, is operated 
to release the apparatus at all stations, restoring the circuit to its 
normal condition. 



PARTY LINE SYSTEMS. 449 

We come, now, to the third general method of selective signal- 
ing pointed out in the beginning of this chapter wherein the selec- 
tion from among the various stations is made by means of ringing 
currents of different frequencies. These systems, with one excep- 
tion, make use of the fact that every pendulum or every vibrating 
reed has a natural period of vibration, and that it can be made to 
take up this vibration by the action of a succession of impulses of 
force occurring in the same frequency as that at which the reed 
or pendulum vibrates. A familiar example of this is found in one 
person pushing another in a swing. The swing has its natural 
period of vibration depending on the length of the ropes, and a 
gentle push applied at proper intervals by the person on the ground 
will cause the swing to vibrate with considerable amplitude. If 
the pushes are applied at intervals not corresponding to the nat- 
ural period of vibration of the swing, many of them tend to retard 
rather than help its vibrations, so that a useless bumping results, 
producing but little motion. 

The utilization of this principle has given inventors a very at- 
tractive field of work; but as in the case of the step by step sys- 
tems, the results attained have, until very recently, proved of little 
practical value in telephony, save in so far as they have contributed 
to the general stock of knowledge on the subject. 

This idea was used in telegraphy before the birth of telephony. 
A number of currents of different rates of vibration were impressed 
upon the circuit by as many different transmitters, each particular 
rate of vibration being capable of operating a reed in one of the 
receiving instruments, and producing no effect upon the others. 
By this means each receiving instrument was capable of picking 
out only those signals sent by the transmitter having the same 
rate of vibration, and thus all of the transmitters could be used 
simultaneously in the same circuit, producing a system of multiplex 
telegraphy. 

The plan of harmonic signaling on party lines was proposed be- 
fore the year 1880 by Messrs. Currier and Rice. They used magnets 
having armatures tuned to vibrate at certain fixed rates, one of 
these magnets being placed in series in the line at each station. 
All these magnets were therefore subjected to whatever current was 
flowing in the line. The armatures, or reeds, as they may be 
called, at the different stations, all having different rates of vibra- 
tions, did not respond unless the current on the line was of the 
corresponding frequency. Thus, by throwing a current of the 

29 



450 AMERICAN TELEPHONE PRACTICE. 

proper frequency on the line, a reed at any one of the stations could 
be thrown into vibration. When so vibrated it served to close 
the circuit through a call-bell at that station, which was, therefore, 
operated. 

Nearly every conceivable form of circuit arrangement was tried 
during the succeeding years until nearly the present date, the work 
of Messrs. Elisha Gray, Frank L. Pope and J. A. Lighthipe stand- 
ing out in some prominence. 

So far as the writer is aware, the only commercial application 
of the harmonic idea of selective signaling on party lines, prior to 
the year 1903, was made by the local Bell Telephone Com- 
pany, of Sacramento, CaL, where a few lines were equipped and 
operated for several years on this principle. Although not an 
unqualified success, this single application served to show that the 
harmonic principle was feasible if it should ever be properly worked 
out. During the past two years Mr. W. W. Dean has applied 
himself to this problem with the result that a four-party selective 
system has been produced, operating on the harmonic principle, 
which has already been put into wide use by the Kellogg Com- 
pany, notably in the system of the Frontier Telephone Company, 
of Buffalo, and which gives every indication of proving an entire 
success. 

Nearly all previous attempts at the solution of this problem in- 
volved the use of relays with tuned armatures, so arranged that 
when an armature was thrown into vibration it would close the 
circuit of the bell, and thus in one way or another cause its oper- 
ation. 

The work of Lighthipe seems to have been one of the exceptions 
to this general rule, and he caused the tuned armature itself to play 
between the two gongs which were to produce the sound. Light- 
hipe bridged his bells directly across the circuit of the line, as in 
the bridging system, a condenser being included in circuit with 
each bell. The same general plan has been followed by Dean, but 
he uses one gong instead of two and a polarized responsive device 
instead of the usual non-polarized electro magnet. The circuits 
of Mr. Dean's system are shown in Fig. 331. 

The cord circuit shown in this figure is a simplified Western 
Electric circuit, having four ringing keys, which are adapted to 
connect the different frequencies of alternating currents with the 
tip side of the line, at the same time grounding the sleeve side. 
The system is adaptableto any switch-board circuits. At each sub- 



■i*$ 






Hi 




INS. 



^ 



■art): 



MX 



451 



452 



AMERICAN TELEPHONE PRACTICE. 



scriber's station, besides the usual talking apparatus, the bell mag- 
net, with its tuned reed and bell tapper, is bridged across the line 
in series with a condenser, C. Currents of 2000, 4000, 6000 and 
8000 alternations per minute are used, these currents being sup- 




FIG. 332.— KELLOGG PARTY-LINE BELL. 



plied by different generators properly governed to maintain a con- 
stant speed. 

The bell mechanism is shown complete in Fig. 332, two views 
being given. The whole mechanism is mounted on a horizontal 
iron plate, d, the bell being mounted in a horizontal position be- 
low the plate, and adjustable toward or from the tapper in 
the usual manner by means of a pivot lever and set screw, 



PARTY LINE SYSTEMS. 



453 



as shown in the detail, Fig. 333. The polarized electro magnets 
consist of two bobbins of the usual form, mounted on a yoke-piece, 
y, at the rear, and having their forward ends supported firmly in 
a brass plate, q, secured at its lower end to the plate, p. Two slotted 




FIG. 333.-DETAIL OF KELLOGG PARTY-LINE BELL. 

pole pieces, s, s, of soft iron are adjustably secured to the front 
ends of the cores of the magnets in such a manner as to allow the 
armature, a, to freely vibrate between them. 

The vibrating part of the mechanism, including an armature, a 
ball-striker and two vibrating springs, are shown in detail in the 
right-hand portion of Fig. 334, together with their supporting 




FIG. 



-DETAILS OF ARMATURES AND STRIKERS, KELLOGG 
PARTY-LINE PELL. 



pieces. In the left-hand portion of this figure four such armature 
units, having different sizes of ball strikers, are shown. 

The armature proper is made of two pieces of soft iron, b b, riv- 
eted together and carrying between them a heavy spring, c, which 



454 AMERICAN TELEPHONE PRACTICE. 

is secured at its upper end to the metal support, a, and at its lower 
end a small spring, e, carrying a split-ball, /. This whole vibrating 
part is secured permanently together by riveting, and the support, 
a, is mounted on the upper portion of the vertical plate, q, as clearly 
shown. The heavy permanent magnet is secured to the iron yoke, 
y, at the rear of the magnets, and projects forwardly so as to im- 
part its magnetism to the upper portion of the armature. It will 
be seen, therefore, that the magnetic design of the mechanism is 
along the line of the Siemans relay. By means of the permanent 
magnet the two pole pieces of the cores of the electro magnet are 
given one polarity and the armature is given the other. The cur- 
rents passing through the two windings will strengthen one pole 
piece and weaken the other, thus causing the attraction of the arma- 
ture toward one or the other of the pole pieces. The reversal of 
the current will cause an attraction toward the other pole piece. 
The armature will, therefore, tend to vibrate in unison with any 
vibratory current which is sent through the magnet, and would do 
so but for the fact that it is so designed as to have a certain distinct 
period of vibration of its own. 

It will be seen that the ball-striker carried on the armature en 
gages the inside of the signal gong rather than the outside, and that 
the gong is adjustable toward or from this striker by virtue of its 
being mounted on a lever carried on the upper side of the plate, 
d, as shown in detail in Fig. 333. The only difference in the vari- 
ous stations on the line is in the size of the ball-striker which en- 
gages the bell. The relative sizes of these balls for the four differ- 
ent stations are shown in the left-hand portion of Fig. 334. It is 
quite evident that a small ball so mounted will vibrate more rapidly 
than a large one, and the balls have therefore been proportioned 
so that in operation they will respond respectively to alternations 
taking place, 2000, 4000, 6000 and 8000 times a minute respectively. 

Mr. Dean found that the placing of the bell or gong in such a 
position as to be struck by the ball changed the natural rate of 
vibration of the ball. For this reason he tuned the device so that 
the proper rate was secured when the bell or gong was in place, 
rather than tuning with respect to the vibration of the reed alone, 
as had been the case in most former efforts in this direction. 

He also found that a polarized device was absolutely necessary 
in order to secure the proper promptness in response, and that a 
flexibility of connection between the armature and the ball was 
also necessary in order that the contact of the ball might not damp- 




155 



456 



AMERICAN TELEPHONE PRACTICE. 



en and destroy the vibration of the armature. He used one gong 
instead of two because he found it impossible, when using two 
gongs, to make the ball strike more than one of them with any 
degree of certainty. 

The machine for supplying the currents of the various frequen- 
cies in ringing in this system is shown in Fig. 335. In this four 
generators are directly coupled on the same shaft, all with a single 





FIG. 336.— GOVERNOR FOR RINGING MACHINE, KELLOGG 
PARTY-LINE SYSTEM. 



motor, this motor serving to drive all four generators. The motor 
is the center one of the machines shown. The generators are re- 
spectively of the two-pole, four-pole, six-pole and eight-pole type, 
and being driven at the rate of 1000 revolutions a minute, therefore 



PARTY LINE SYSTEMS. 457 

produce currents having frequencies of 2000, 4000, 6000 and 8000 
alternations per minute. 

In order to properly govern the speed of the motor so that it 
will not fluctuate with changes in the primary voltage, a gov- 
ernor, shown in diagram and perspective in Fig. 336, is 
mounted upon the end of the motor shaft. This consists of a 
ball, b, mounted by means of a spring on the armature shaft. This 
spring is adapted to make contact with the point, o, when the ball 
is thrown outward by centrifugal force. The method of govern- 
ing is as follows: With the desired rate of revolution, 1000 per 
minute, the motor was so constructed as to run at 900 revolutions 
on the highest voltage it could ever get. A resistance was then 
introduced in the field circuit of the motor sufficient to make it run 
at 1 100 revolutions on the lowest voltage it could ever get. The 
connection was so made that the contacts on the governor would 
short-circuit this resistance when the ball is forced out so as to 
close it. It is evident, therefore, that normally a resistance is in 
circuit in the field, tending to make the motor run too fast. As 
soon as the contact is closed, however, by centrifugal action on the 
ball, the resistance is short-circuited, tending to make the motor 
run too slow. By adjusting the tension of the spring the motor 
can be made to regulate at within one per cent, of 1000 revolutions 
a minutue when subjected to a fluctuation .in primary voltage of 
over 100 volts. 

There is one other "frequency" system of party-line working 
which deserves attention. This is the Leich system, as put on the 
market by the American Electric Telephone Company, of Chicago. 
This system depends for its action on the fact that a high frequency 
current is more readily transmitted through a condenser than one 
of low frequence, while the reverse is true with respect to trans- 
mission through an impedance coil. 

The circuits of a Leich four-party line and a central office cord 
circuit is shown in Fig. 337. In this two generators of ringing 
current are employed at the central office, one giving a frequency 
of 2400 alternations per minute and the other of 7200. Four ring- 
ing keys are so arranged as to connect either the high or the low 
frequency machine between either side of the line and ground. Two 
of the bells of the four sub-stations are bridged between one limb 
of the line and ground, the other two being similarly connected 
with respect to the other limb. Each bell has associated with it a 
retardation coil and a condenser, the arrangement being such that 



458 



AMERICAN TELEPHONE PRACTICE. 



one of the bells on each side of the line will respond only to cur- 
rent from the high frequency generator, and the other only to that 
from the low frequency generator. 

As will be seen from the circuit stations, I and 3 are high-fre- 
quency stations, and at each of these the bell is shunted by a low- 
wound impedance coil, this parallel circuit being placed in series 
with a one-microfarad condenser. The bells at stations 2 and 4 
are adapted to respond to low frequency currents only, and there- 
fore are placed in series with a two-microfarad condenser and a 



HlQH 
FREQUENCY 



hi«;h 

FREQUENCY 



l-OW 

FREQUENCY 



FREQUENCY 




FIG. 337.— LEICH PARTY-LINE SYSTEM. 



high-wound impedance coil. Each limb of the line carries one 
high and one low frequency station. 

If the low frequency generator is connected with the limb, a, of 
the line, by throwing ringing key, 2, some current will pass to 
earth through the bell of both stations 1 and 2. Most of the cur- 
rent that passes to ground at station 1 will, however, pass through 
the shunt coil around the bell, and, moreover, the low-capacity con- 
denser proves a somewhat serious barrier to the slowly fluctuating 
current. On the other hand, all of the current that passes to ground 
at station 2 passes through the bell magnets, This current is of 



PARTY LINE SYSTEMS. 459 

sufficient magnitude to operate the bell, the larger capacity con- 
denser allowing the low frequency current to pass through it more 
easily than that at the other station. The bell at station 2 rings, 
therefore, while that at station 1 does not. 

If ringing key, 1, is thrown, the high frequency generator will be 
connected to limb, a, of the line, and, as before, some current will 
flow to ground through the bells of stations 1 and 2. In this case 
the combined impedance of the 1200-ohm coil and the ringer mag- 
nets at station 2 to the rapidly alternating current, will prevent 
enough current passing to ring the bell at that station. At station 
1, the retardation coil will shunt enough current through the bell 
to cause it to ring, the one-microfarad condenser allowing the high 
frequency current to pass through it much more readily than the 
low. The operation of the two stations on the limb, b, of the line 
is the same. 

The operation of the Leich system may be said to depend on a 
system of electrical tuning, the bell circuits at each sub-station being 
tuned to be responsive to either high or low frequency currents by 
means of the arrangement of condensers and impedance coils. 



CHAPTER XXV. 
MEASURED SERVICE. 

Methods of charging for telephone service are now the subject 
of much discussion, and it is not improbable that in the near future 
they will undergo general and radical changes. The plan that has 
been generally adopted is to charge the subscriber a fixed sum, 
payable monthly or quarterly, for the regular exchange service ren- 
dered, regardless of whether he uses his telephone much or little. 
Under this arrangement a subscriber who used his telephone once 
a day would be charged the same amount as one who used it fifty 
times a day. This is called the flat-rate plan, and its disadvantages 
have become more and more serious with the ever-increasing use 
of the telephone. 

Besides being an inequitable arrangement, the flat-rate plan per- 
mits of the telephone being used for trivial purposes, which would 
not be the case if the subscribers knew that each message cost them 
a certain fixed amount of money. 

Again, under the flat-rate plan people who have no telephone, 
and who cannot afford to pay the high rate usually charged for flat- 
rate service, make use of their neighbors' instruments, thus securing 
partial advantage of the telephone service without cost to them- 
selves. This class of business, which has been properly termed 
"dead head," is disadvantageous to the operating company, because 
it is forced to supply operators and switch-board equipment for 
handling the increased traffic; and it is disadvantageous to those 
subscribers who do pay rental, on account of the time their line 
and instrument is used for handling business other than their own. 

During recent years telephone companies have gradually awak- 
ened to the fact that to handle a telephone call and connection costs 
money; that the cost of operation increases with the traffic. Not 
only are more operators required when the average traffic on the 
lines increase, but a greater number of switch-board sections are 
made necessary in order to provide room for the increased number 
of operators. With a more complete realization of this condition, 
there has been a strong tendency toward supplying a different class 

460 



MEASURED SERVICE. 461 

of service under such conditions that the subscriber will pay in ac- 
cordance with what he gets. This, of course, involves some plan 
of measuring the amount of service rendered the subscriber, so that 
the telephone business will be reduced to practically the same basis 
as to charge as that of gas and electric-lighting companies. 

The problem of measuring telephone service is, however, a differ- 
ent one from that of measuring the number of cubic feet of gas, or 
the number of watt-hours used in electric lighting. The question 
to be solved first in the telephone business is, what is the proper 
commodity to measure. The simplest plan is to measure the num- 
ber of times a subscriber makes a call, but this has the disadvantage 
of being inequitable, because many times the calling subscriber 
will not obtain a connection, owing to the fact that the subscriber 
called for is busy at the time, or that for some reason or other he 
does not answer. Seemingly a fair plan is to charge the sub- 
scriber for the number of successful connections obtained as a result 
of the calls he originates. This is the basis upon which most of the 
successful systems for measuring telephone service are based. 

There have been those who contended that a subscriber should 
be charged not only for those calls he originates, but also for those 
calls he receives; but this plan does not work out to the satisfac- 
tion of the public, because a subscriber has no means of regula- 
ting the number of times he is called by the public, and there are 
few who would wish to place themselves in a position to pay for 
that over which they have no control. To charge on the basis of 
the number of calls received would be analagous to charging, in 
the postal system, for the number of letters received. Under cer- 
tain circumstances, as in the case of a philanthropist, this might 
become a serious burden. 

The first class of devices for measuring telephone service to be 
considered will be those which may be termed "coin-collecting 
devices," whereby the subscriber is made to pay as he goes, always 
depositing a coin in the collector before actually conversing with 
the party called for. 

This has the advantage to the operating company of making 
collections in advance. Also, from the standpoint of the telephone 
company, and perhaps of the subscriber, it has the advantage of 
allowing the payments to be made in such small installments as 
not to appreciably burden even the poorest subscriber. Such de- 
vices are, however, subject to the disadvantage of requiring frequent 
visits to the premises of the subscriber to collect the money de- 



462 AMERICAN TELEPHONE PRACTICE. 

posited, together with the incidental disadvantage due to the pos- 
sibility of fraud on the part of the collectors and of the public. 

Probably the simplest coin collectors are those which have no 
electrical connection whatever with the telephone or line circuits. 
These are, therefore, entirely mechanical in their operation, and 
are so arranged as to make a certain distinctive noise when a coin 
is deposited in the proper manner. This noise is transmitted to the 
head receiver of the operator at the central office through the trans- 
mitter at the subscriber's station in the same manner and over the 
same circuits as are used in speech transmission. 

Among the strictly mechanical coin collectors is that manufac- 
tured by the Baird Manufacturing Company, of Chicago. An out- 




FIG. 338.-BAIRD COIVCOLLECTING DEVICE. 

side view of this is shown in Fig. 338. The box proper is of cast 
iron, provided on its lower front face with a door for allowing access 
to the coin box. Three slots appear in the upper front face of the 
box for the reception of 5, 10 and 25-cent coins, respectively. At 
the right of the box, adjacent to the slots, is a lever which, when 
pulled down after the insertion of a coin in a slot, sounds a gong 
or equivalent device, giving a significant noise, differing for each 
slot. By means of this the operator is notified by telephone that 
the coin of a certain denomination has been deposited. Pulling the 
lever down without depositing the coin does not make the required 
noise, the coin itself being necessary for the operation of the sound- 
ing device. In order to enable the operator to distinguish with as 



MEASURED SERVICE. 463 

great a degree of certainty as possible, the sounds caused by the 
operation of the levers are made to differ as radically as possible. 
To this end the 5-cent lever when operated causes the striking of 
a deep toned gong, such as is used in cathedral clocks, this gong 
consisting of a coil of heavy steel wire. The 10-cent lever causes 
a rasping or buzzing noise due to the vibration of a steel spring, 
which strikes in its vibration against the side of the box. The oper- 
ation of the 25-cent lever causes the striking of a high-toned gong, 
similar to those used in telephone ringers. 

Should the coin be dropped by mistake into the wrong slot, as 
for instance, the nickel into the 25-cent slot, or the 10-cent piece 
into either the 5 or the 25-cent slot, or a penny into either of the 
last-named slots, no effect will be produced and the coin will pass 
through the machine and out at the pocket shown at the left-hand 
side of the cut where the depositor may reach it. All coins which 
are properly deposited are thrown by the operation of the lever 
into the coin box below, where they remain beyond the reach of the 
subscriber, and only accessible to the collector. 

The door of the coin box is secured by an ingenious combination 
lock, requiring a very simple key for its manipulation, security 
being, however, dependent upon a proper knowledge of the com- 
bination rather than upon the shape of the key. 

Several methods of attaching these collectors to telephone sets 
are shown in Fig. 339. At the right a heavy sheet-iron plate 
carrying the coin box is secured to the rear of the back board. 
In these purely mechanical coin collectors, care must be taken in 
installation to make as good as possible the conditions for conduct- 
ing to the telephone transmitter the distinctive noises made by the 
deposit of coins. In the style of mounting shown at the right in Fig. 
339 the conditions are improved by fastening the transmitter with 
machine screws passing through the wood of the telephone set and 
threaded directly into the iron plate holding the coin collector. In 
the center is shown a method of mounting, in which the transmitter, 
with a special bracket, is attached directly to the case of the coin 
collector. In this style of assembly, the noises are very plainly heard 
by the central office operator. An adaptation of this type of coin 
collector to desk portables is shown at the left of Fig. 339. 

When one of these coin boxes forms a portion of a sub-station 
equipment the subscriber originates his call in the usual way. either 
by turning his generator crank in the magneto systems, or by re- 
moving his receiver from the hook in the case of a common bat- 



MEASURED SERVICE. 



465 



tery system, and makes known to the operator the number of the 
subscriber with whom he desires a connection. The operator then 
makes the connection with the called subscriber, and upon securing 
his response notifies the calling subscriber to deposit a coin of the 
proper denomination. She is notified of his compliance by hear- 
ing the characteristic sound of that coin, after which she allows 
the conversation to progress. 

An extensive line of purely mechanical coin-collecting devices is 




FIG. 340.— STROUD CONTROLLER FOR STREET SERVICE. 



manufactured by the Gray Telephone Pay Station Company, of 
Hartford, Conn. In these, coins dropped through the slots are, 
if of the proper denomination, caused to strike against one or more 
gongs, the sound being transmitted to the operator as before. 
Some of these have been made with as many as five slots, corre- 
sponding to nickels, dimes, quarters, halves and dollars. Each 
coin when dropped through its proper slot strikes against the gongs 
underneath in such manner as to convey a distinctive signal to 
the operator. The code of signals in such boxes is: A single bell 



466 



AMERICAN TELEPHONE PRACTICE. 



means a nickel; two bells a dime; one clock gong, 25 cents; two 
clock gongs, 50 cents, and a bell and rattle, $1. 

Other devices have been used where the coin, after being dropped, 
closed a succession of electrical contacts, which, acting through the 
primary of the induction coil at the subscriber's station, caused a 
series of buzzes which may be distinguished by the operator. 

In Fig. 340 is shown a public pay station, manufactured by the 
Controller Company of America. This is ihe design of Mr. Harold 
D. Stroud, and is adapted to street service, the box being of iron 
and weatherproof. 

The coin-collecting devices so far considered have, as a type, 





FIG. 341— SCRIBNER COIN-COLLECTING DEVICE. 



the advantage of simplicity. All of these boxes, however, impose 
a duty upon the operator which necessarily tends to slow down her 
speed of working, thus producing what is called a "drag" on her 
work. The fact that the operator, after receiving a call, is required 
to obtain the proper connection, and then again place herself in 
communication with the calling subscriber before allowing the con- 
versation to proceed, is a disadvantage which increases with the 
amount of traffic, and becomes a very serious matter in exchanges 
where the operators are already loaded with as much work as they 
can handle. More recent developments in the line of measured 
service have, therefore, aimed at a reduction in the drag on the 



MEASURED SERVICE. 



467 



operator. Without attempting to trace through the development 
by which coin-collecting devices for telephone stations have reached 
their present state of perfection, two systems and devices which are 
thought to represent the highest development in this class of work 
will be described. 

The first of these is a box, designed by Scribner for the Western 
Electric Company, and used very widely among the Bell companies 
at present. The Chicago Telephone Company is now the foremost 
advocate of the measured service plan, and this company alone 
employs about 38,000 of these boxes in its Chicago exchange. 
While the particular box shown is the work of Scribner, many other 




342.— SCRIBNER COIN-COLLECTING DEVICE. 



of the Western Electric and Bell inventors did much of the pre- 
liminary work leading up to the type about to be discussed. Among 
these the work of O'Connell and Bullard is conspicuous. 

The details of the Scribner box are shown in Figs. 341, 342 and 
343. In the right-hand portion of Fig. 341 a front elevation of 
the box with its cover removed is shown, the cover with the de- 
posit and return slots and key-hole being shown at the left of this 
figure. Fig. 342 shows a sectional view taken on the dotted line, 
3-3, of Fig. 341. Fig. 343 is a sectional view looking from the 
top of the box, this being taken on the plane of the horizontal dotted 
line, 5-5, of Fig. 341. Fig. 343 also shows a perspective view of 
the coin-controlling magnets. 



468 



AMERICAN TELEPHONE PRACTICE. 



The upper slot in the cover of the box, shown in Fig. 341, leads 
into the portion, a, of the coin chute, shown most clearly in Fig. 
342. The coin passes by gravity through the upper portion of 
this chute and is led to the rear of the box, and then, owing to the 
form of the chute, again toward the front of the box. Under nor- 
mal circumstances it strikes against stationary pin, a 5 , and rolls 
back into the positon shown in Fig. 342, resting between the pins. 
e' and e 2 . These two pins are carried on the pivoted armature, 
e, of the electro-magnet, f, this armature being a flat piece of iron 
of approximately hexagonal shape, pivoted on its vertical axis by 
ordinary trunnion screws, which are shown most clearly in Fig. 
341. The electro magnet, f, is polarized in much the same man- 







~ " . J 



-6 



343.— SCRIBNER COIN-COLLECTING DEVICE. 



ner as an ordinary ringer, and therefore when its coils are traversed 
by a current in one direction the armature will be tilted so as to 
withdraw one of the pins, e' or e 2 , from the chute and to push the 
other one further into it. A current in the reverse direction will 
tilt the armature the other way. As the deposited coin is sup- 
ported in the chute under normal conditions by these two pins, 
it is evident, referring again to Fig. 342, that if the pin, e', is with- 
drawn, the coin will roll from its normal support and pass through 
the return chute, a 2 , into the receptacle, b, on the front of the box. 
If, however, the pin, e 2 , is withdrawn, the coin will roll in the oppo- 
site direction and will pass through the hole, a 3 , and be guided into 
the cash box. It is obvious, therefore, that a current sent through 



MEASURED SERVICE. 469 

the electro magnet, f, in one direction will return the coin to the 
user, while a current in the other direction will collect the coin. 

If, while a coin remains lodged between pins, e' and e 2 , a subse- 
quent coin is dropped into the box, it will roll over the top of the 
first coin and thus pass over the pin, a 5 , and to the left of that pin, 
as shown in Fig. 342, to the return chute. , This prevents clogging 
of the box in case a coin remains undisposed of. 

Referring now more particularly to the perspective view shown 
in Fig. 343, it will be seen that the pin e' is permanently fixed to 
the armature plate, e, while the pin e 2 is pivoted in the plate so 
as to swing sidewise. A spring, i, resting against a laterally pro- 
jecting arm of the pin e 2 serves to keep this latter pin normally 
pressed toward the pin e' . When, however, a coin is lodged be- 
tween the two pins, the pin e 2 is moved to the right so as to make 
contact with the pin e 3 , which is insulated from the armature plate, 
but is in connection by the strip, k, with the spring, h, shown at the 
top of the figure. The spring, h, forms one terminal of the cir- 
cuit, and the frame of the machine the other terminal, and this cir- 
cuit is closed temporarily.by the deposit of a coin. 

The operation of this box will now be best understood by refer- 
ence to Fig. 344, which shows the circuit of the subscriber's line, 
extending to a central office equipped with a multiple board of the 
well known Western Electric type. The only modification of the 
line circuit at the central office from that ordinarily employed is that 
the contact of the cut-off relay on the ring side of the line is left 
open and the contact on the tip side of the cut-off relay is connected 
to the line relay instead of being connected to ground. Two keys, K 
and K\ similar to ringing keys, are associated with the answering 
cord of each pair, as shown. At the subscriber's station, in addition 
to the ordinary common battery telephone set, the coin box is placed, 
the electro magnet, f, of this box being connected between the tip 
side of the line and ground. The circuit from the ring side of the 
line to the magnet includes also the pair of contacts, e 2 , c 2 , closed by 
the deposit of the coin. 

A subscriber desiring to make a call does so, not by raising his 
receiver from its hook, but by dropping a coin of the proper de- 
nomination, usually a nickel, in the slot. This coin rests between 
the pins, e' and c 2 , of the box, and closes the contact to ground 
through the magnet, /, as shown diagrammatically in Fig. 344. This 
completes the circuit through the line relay, using ground return, 
and the line lamp is lighted in the usual manner. The current flow- 



470 



AMERICAN TELEPHONE PRACTICE. 



ing over the tip side of the line does not operate the magnet, f r 
in either direction because it is too feeble to do so. 

In response to the line signal the operator inserts an answering 
plug, thus cutting off the circuit through the line signal by means 
of the cut-off relay. The operator then converses with the sub- 
scriber, calling in the usual manner, and completes the connection 




FIG. 344.— CIRCUIT OF SCRIBNER COIN-COLLECTING SYSTEM. 

between his line and that of the called subscriber by means of the 
calling plug. When the called-for subscriber responds, the oper- 
ator, noticing the going out of the calling supervisory lamp, presses 
the key, K, which interrupts the conversation between the two sub- 
scribers, if it has begun, and connects the negative pole of a no- 
volt battery with the tip side of the line. The current thus flows 
from ground at central office, through this side of the line to ground 
through the magnet, f, of the coin device, which latter magnet is 



MEASURED SERVICE. 



471 



then actuated to withdraw the pin e 2 from under the coin, throwing 
the coin into the cash box. If, however, the called-for subscriber 
does not respond, or if his line is busy so that the operator cannot 
at the time make a connection with it, she depresses the key, K" 
which connects the positive pole of the no-volt source of current 
with the tip side of the line, thus causing 1 current to flow through 
the magnet, /, in such direction as to withdraw the pin e' from 




FIG. 345.-STROUD COIN-COLLECTING DEVICE. 



beneath the coin, allowing it to roll into the return chute, and back 
to the depositor. 

This device serves to measure the service on a strictly equitable 
basis, save for the fact that no discrimination is made between a 
long conversation and a short one, as a subscriber's money is re- 
turned to him in case he does not receive the connection for which 
he calls. The fact that the subscriber is forced to deposit his money 
before he can make the call is of great advantage from the stand- 
point of the operating company, for the operator knows that the 



472 AMERICAN TELEPHONE PRACTICE. 

coin has been deposited from the fact that a call was received. 
There is, therefore, a much less drag on the operator than if she 
were compelled to request the subscriber to deposit a coin. This 
device also insures the subscriber having the proper denomination 
of coin ready before he makes the call, which often causes much 
delay when the "'request" plan of calling is employed. 

As employed by the Chicago Telephone Company, however, the 
full advantages of this system do not seem to be obtained. As used 
by this company, the return side of the line is grounded at the cut- 
off relay, as usual, and therefore the subscriber calls central in the 
usual manner, that is, by raising his receiver from the hook. He 
drops his nickel when told to do so by the operator, immediately 
after telling her of the connection desired. After this the operation 
of returning or cashing the coin is the same as that already pointed 
out. The lighting of the lamp, L, when the operator presses in her 
key, K or K' , shows the operator that the coin has been deposited, 
while its failure to light on subsequent pressure of the key, shows 
her that the circuit has been cleared. 

In Fig. 345 is shown a coin-collecting device, designed by H. D. 
Stroud, of the Controller Company, which has the advantage over 
the Western Electric device of still further reducing the drag on the 
operator. In this the coin is deposited by the subscriber in order 
to make the call, and when so deposited it is arrested in its chute 
in front of a small glass window in the center of the cover. In 
this position the coin is held in view of the public until the next 
call is made. When the operator responds she completes the con- 
nection as called for by the subscriber in the usual way, and if the 
proper party is secured she has no duty other than would be im- 
posed by the ordinary flat-rate service. The only contingency 
which may arise to require a special movement on the part of the 
operator is in case she does not secure the proper party, in which 
event she presses a key associated with the answering plug which 
operates the coin-controlling magnet to return the coin to the sub- 
scriber. In case the called-for party is secured, the coin is left in 
front of the window in the box, and is deposited by the falling of 
the next deposited coin. The electro magnet is not polarized, which 
is an advantage in the point of simplicity. The holding of the coin 
in front of the window also has a tendency to prevent the illegiti- 
mate use of slugs to operate the device, as the holding of the slug 
in view of the public until the next call is made is likely to expose 
the fraud. As by far the greater number of calls made are ter- 



MEASURED SERVICE. 



473 



minated successfully, it follows that in most cases the operator 
has no duties to perform in the operation of the Stroud device other 
than those imposed by ordinary flat-rate service. In the small 
percentage of calls, however, which are not successful, she presses 
the return button, which is the only extra duty imposed on the 
operator as the result of this service. This represents the highest 



SOB"5 




o^£-/"^L 


M 




g* — 


<^ 






* ^~itr r 




H 




— vwv- 





FIG. 346.-CIRCUIT OF STROUD COIN-COLLECTING SYSTEM. 



development of coin-collecting devices that has been brought to the 
attention of the writer. 

One form of circuit arrangement proposed by Stroud for use with 
his coin collecting device is shown in Fig. 346, this representing the 
system as applied to the Western Electric cord and line circuits. 
The restoring magnet, A, of the coin box is included in series in 
the line circuit at the sub-station, and for this reason it is made of 
low resistance and is provided with a heavy copper covering lor 
the core and with heavy copper spool heads for the magnet winding. 



474 AMERICAN TELEPHONE PRACTICE. 

By this construction the magnet is made to have a very low co- 
efficient of self-induction so as not to materially interfere with the 
talking efficiency. A pair of contacts, B, is adapted to make a 
passing contact between the ring side of the line and ground when 
a coin is deposited in the slot. 

At the central office the line circuit is so modified that the tip and 
ring side are normally connected together by means of the left- 
hand pair of contacts on the cut-off relay, C. The line relay, L, is 
connected permanently to the battery, the other terminal of its 
coil being connected to the ring side of the line through the right- 
hand pair of contacts of the cut-off relay. When the cut-off re- 
lay is actuated by the insertion of a plug by the operator the 
connection between the two sides of the line is broken, as is also 
the normal connection between the line and battery. The line relay 
is so connected as to have its coil included in its own local circuit 
with the line lamp and the right-hand pair of contacts on the cut-off 
relay. By this means the line relay locks itself up as soon as actu- 
ated by current flowing over the line, and then keeps the line lamp 
illuminated until the locking circuit is broken by the action of the 
cut-off relay. 

In this system a subscriber may take his receiver off its hook 
without getting response from the operator until he has deposited 
his coin. The closing of the contact at B by the passing of the coin 
then completes a circuit from the live side of the battery through 
the line relay coil and thence over the two sides of the line in mul- 
tiple to ground through the contact at B. Although the contact at 
B is immediately opened the line relay remains operated, thus keep- 
ing the line lamp illuminated until the response of the operator. 
If the subscriber desired is available and the connection made, the 
operator pays no more attention to the connection than she would 
in flat rate service, as she knows that the subscriber must have de- 
posited his coin before he could light the line lamp. She knows 
also that the coin will be thrown into the cash box by the action of 
the next deposited coin. If, however, she cannot obtain the con- 
nection desired, she will depress the key, K, and thus by means of 
current from a no-volt source cause the energization of the return 
magnet, A, at the sub-station. This magnet is too insensitive to 
be operated by current from the 24-volt storage battery. 

In public telephone stations in a city, the traffic is well divided 
between city calls at 5 cents each, and suburban and long-distance 
calls involving larger amounts, which could be paid more conven- 



MEASURED SERVICE. 475- 

iently in io-cent and 25-cent coins. To use the simple mechanical 
machines described first in this chapter would entail an incidental 
delay and inconvenience on each local 5-cent call, while to use the 
electrical machines would require an attendant, say the druggist or 
clerk, to collect the larger amounts for toll line connections. Of the 
two conditions, the policy of the Chicago Telephone Company is to 
use the electrical machine generally, with exceptions in favor of the 
mechanical devices when an attendant for collecting the larger 
amounts is not available. For use at such stations Mr. David S. Hul- 
fish has produced recently, for the Baird Manufacturing Company, 
models of coin collectors in which the electrical equipment for the 
rapid handling of local business is' combined with the necessary slots 
and audible signal devices for collecting coins mechanically when 
required for suburban' and toll line connections, the electrical fea- 
tures being such as to work indiscriminately with single-slot ma- 
chines of the Western Electric or Stroud types described. 

There is obviously another general method for charging for tele- 
phone service in proportion to the use of the telephone, than by 
the collection of coins or tokens at the subscriber's station. Meters 
may be employed which automatically keep a record of the number 
of times that the telephone is used, and these meters may be placed 
either at the subscriber's station or at the central office. 

Considering first the placing of a meter at the subscriber's sta- 
tion, it may be said that the problem is very similar to that of 
collecting the coins, differing only in that some other function on 
the part of the subscriber than that of dropping a coin is used to 
operate the meter. A device has been produced by William Gray, 
of the' Gray Telephone Pay Station Company, which operates on 
the same general plan as the Baird coin collector except that, in- 
stead of dropping a coin and pulling a lever as in the Baird and 
other similar devices, the subscriber, when told to do so by the 
operator, inserts a key into a keyhole in front of the box and turns 
it. The turning of the key not only operates the meter to register 
one count, but also trips a lever carrying a hammer which strikes 
a gong, and this notifies the operator by sound that the meter has 
been actuated. This meter has no electrical connection whatever 
with the system, its entire function being mechanical. 

In another device manufactured by the Gray Company no key 
is required, the subscriber merely pressing aTmtton on the front of 
the box when told to do so by the operator, this action ringing the 
bell and registering one count on the meter. One of these devices 



476 AMERICAN TELEPHONE PRACTICE. 

is shown in Fig. 347. The method of securing this to a portable 
desk stand is shown in Fig. 348. 

These devices are simple and effective, but are subject to the same 
disadvantages as those coin collectors which have no electrical con- 
nection with the central office, the principal one of which is the 
amount of attention required by the operator to assure the proper 
registration of the meter. This fault is largely remedied in a class 
of meters which, like the Scribner and Stroud coin collectors, are 
partially controlled by the central office operator. 

Prominent in this class is the device of H. V. Hayes, which 




FIG. 347.-GRAY PUSH-BUTTON METER. 

consists of an electro-magnetically operated meter placed at each 
subscriber's station, the operating coil of the meter being placed in 
a bridge of the telephone line which includes the talking apparatus. 
This device is adapted to use on common battery lines, and there- 
fore the coil of the meter receives current whenever a subscriber 
removes his receiver from its hook for the purpose of sending a 
call. One impulse of the armature lever, however, will not affect 
the registration on the meter dial. In order to cause a registration, 
therefore, two impulses are required, one in one direction and one in 
the other, these impulses always occurring in the same order. For 
accomplishing this, the armature of the controlling magnet is polar- 
ized so as to move in one direction when the coil is traversed bv a 



MEASURED SERVICE. 477 

current of one polarity, and in the other when the current flows in 
the opposite direction. The source of current normally in the line 
which operates the line relay when a subscriber removes his receiver 
from its hook is of opposite polarity from that which is placed in the 
circuit with the line when a plug is inserted for completing a call. 
The first impulse received by the meter when a subscriber makes a 
call is therefore due to the current which flows through the line 
upon the raising of the receiver from its hook. This current places 
the meter into such position that a call will be registered when the 




FIG. 348.— METHOD OF ATTACHING GRAY METER TO DECK-STAND. 

meter lever is moved in the opposite direction, and this occurs 
when the operator answers the call in response to the signal, as 
current then flows from the cord circuit through the meter coil in 
the opposite direction. The meter does not operate on a called 
line because then only the current from the cord circuit battery 
flows through the meter coil, which cannot operate the meter be- 
cause no preliminary current from the line battery has been sent 
through the coil. 

This system of Hayes has the disadvantage of registering a count 
for every call that is answered by the operator, and thus all calls 



•478 AMERICAN TELEPHONE PRACTICE. 

which are not successfully terminated on account of the called line 
being busy, or out of order, or on account of the non-response of 
the called subscriber for any reason, are charged. It has the advan- 
tage, however, of producing no drag on the work of the operator. 
Moreover, it is clearly adaptable to use on common battery party 
lines. 

Later devices have been so arranged that only a successfully ter- 
minated call is charged for, this being accomplished by having the 
operator send the required second impulse through the meter by 
pressing a key after the subscriber called for had responded. This 
relieves the system from the defect of charging for calls other than 
those successfully terminated, but has the same disadvantage as to 
the drag on the operator as that found in the Scribner coin col- 
lector. 

Mr. Stroud has in a large measure remedied this difficulty by 
having the meter register all calls whether successfully terminated 
-or not. In order, however, to prevent false charges, each meter is 
provided with a magnet under the control of the operator which 
when energized serves to turn the meter back one count, thus 
crediting the call. This device has the advantage of requiring 
special work by the operator only when the call is not successful. 
With this scheme party lines could be indiscriminately equipped 
with Stroud meters, or coin collectors, and the operation, so far as 
the central office was concerned, would be identical in each case. 

The external appearance of this meter is shown in # Fig. 349, and 
its operation is as follows: In order to call the central office the 
subscriber pushes the button shown at the top of the meter-box, 
which performs the same function with respect to the line circuit 
as does the dropping of a coin in the slot of the Stroud Coin Col- 
lector — that is, it closes the circuit to ground which causes the line 
relay to pull up, thus lighting the line lamp. The coil of the line 
relay is included in the local circuit of the line lamp so closed, and 
therefore the line relay remains locked until released by the action 
of the operator in plugging into the jack in response to a call. The 
making of a call in this manner registers one count on the meter, 
the record being in plain sight of the subscriber. If the connection 
called for by the subscriber is obtained the operator has no other 
duties in handling it than in the ordinary flat-rate system. If, how- 
ever, the call is not successfully terminated the operator presses 
the button in the answering cord circuit which operates the credit 
magnet in the meter-box, which subtracts one count from the regis- 



MEASURED SERVICE. 



479 



tered number. A modification of this method of crediting for an 
unsuccessful call is employed by the later meters devised by Mr. 
Stroud. In this no arrangement is made to turn the counter back, 
but the equivalent result is accomplished by so arranging the mech- 
anism that the register will not record a count when the subscriber 
makes his next call. In other words, after an unsuccessful call, the 
subscriber is entitled to a free call. In order to satisfy the sub- 
scriber on this point, a small round hole at the left-hand portion 
of the front of the box is provided, at which a white disc is dis- 
played whenever the subscriber is entitled to a free call as the re- 




FIG. 349.— STROUD CREDITING METER. 

suit of his last call having been unsuccessful. This free call attach- 
ment is, of course, controlled by a key in the cord circuit at the 
central office, used only by the operator in the case of an unsuccess- 
ful call. 

Several methods of operating meters at the central office have been 
proposed. Probably the first was to provide in connection with 
each line an electro magnetic counting device, the coil of which 
was included directly in the line circuit or in the local circuit of the 
line relay, this being caused to record one count whenever a sub- 
scriber made a call. This has the obvious disadvantage of sub- 
jecting the subscriber to the condition of over-charging himself 



480 AMERICAN TELEPHONE PRACTICE. 

without being aware of so doing. For instance, the moving of his 
hook up and down in an attempt to attract the attention of the 
operator before a connection has been made with the line would 
record not one but perhaps many counts. As a remedy for this 
defect it has been proposed to operate the meter in connection with 
the cut-off relay at the central office. By this means it is evident 
that a subscriber by manipulating his hook can produce no effect 
upon the meter, which would be operated only when the operator 
inserted the answering plug in response to the display of the line 
lamp. In such cases the meter, in connection with the cut-off re- 
lay of the called line, could be prevented from operating by having 
the current in the calling cord of opposite polarity from that in the 
answering cord, the meter magnets being polarized. This practice, 
however, was subject to the same disadvantage as was pointed out 
in connection with the Hayes meter, as applied to sub-station appa- 
ratus, that is, that all calls answered by the operator were counted 
rather than the successful connections. Another disadvantage of 
less importance was that the meter, being entirely under the con- 
trol of the operator, could be manipulated by her at will, and by 
inserting or withdrawing her plug a few times she could register 
more counts on the meter than should properly be charged. 

Mr. George K. Thompson has proposed a system which has been 
put into operation to some extent by the Bell companies, which 
makes the action of the register dependent upon the conjoined 
action of the subscriber and operator. The method of applying 
this device to the ordinary Western Electric line circuit is shown 
in Fig. 350. The meter shown has two operating coils, A and 
B. The coil A is placed in parallel with the line lamp, D, in such 
manner as to be energized whenever the line relay, C, is energized 
to indicate a -call. The coil B of the meter is placed in multiple 
with the coil of the cut-off relay, N, so as to be operated by current 
from the third strand of the cord circuit whenever a plug is in- 
serted into a jack. 

The meter is so arranged as to register a count only when the 
armature, b, of the magnet B is operated, and this armature cannot 
be operated until the armature, a, of the magnet A has been moved 
out of its way. If the magnet B is operated alone, as is the case 
when a plug is inserted into a jack of a called line, the meter, there- 
fore, will not register a count because the armature b is prevented 
from moving by the presence of the armature a in its path. For 
this reason a call will not be registered on the meter of a called- 



MEASURED SERVICE. 



481 



for line. The armature b carries a pawl which engages a ratchet- 
wheel, properly connected with the train of counters,, which wheel 
is moved one notch by every attraction of the armature, b, of the 
magnet B. In order to prevent the armature a from falling back 
into the path of the armature b, when the operator inserts the an- 
swering plug into the jack of a line upon which a call is being made 
(due to the opening of the circuit of the magnet A by the releasing 
of the line relay due to the action of the cut-off relay), the armature 



r 



SUBS. 
STATION 



jn 



m 






- . a 

b 



FIG. 350.— THOMPSON METER SYSTEM. 



a is made sluggish in its movements, so that the armature b will 
always have time to complete its stroke before the releasing of the 
armature a. 

This device, it will be seen, is subject to the defect that it records 

a count on the meter every time the operator inserts the plug in 

response to a call, whether or not the connection desired is finally 

secured. As a remedy for this defect, Mr. Thompson produced 

3i 



482 



AMERICAN TELEPHONE PRACTICE. 



another system, using the same type of meter, but instead of mak- 
ing the operations of the magnet B depend on the insertion of the 
answering plug into a jack it was made to depend on the action 
of the calling supervisory relay of the pair of cords used, so that 
instead of registering when the answering plug was inserted in re- 
sponse to a call, as in the system shown in Fig. 350, the count would 
be recorded only after the calling plug had been inserted and the 
called subscriber had responded. This involved a somewhat com- 





rE5=H 40- 

f-9 — vm ' 



^D 



FIG. 351.— SCRIBXER METER SYSTEM. 



plicated and probably unsatisfactory condition of circuits depending 
on marginal resistances for securing the operation of the meter, 
and has probably never been put into extensive practice. 

The Bell companies are using, to some extent, a measured ser- 
vice device system, produced by Scribner, employing meters at the 
central office in connection with each line wherein the operation 
of the meter is secured by pressing a push button key associated 
with the answering cords. In this case the meter has a single mag- 
net which is placed in multiple with the cut-off relay in the lead 
from the sleeve contacts of the jacks to ground. The circuits of 



MEASURED SERVICE. 483 

this system are shown in diagram in Fig. 351. The line circuit will 
be recognized as that of the standard Western Electric Company's 
common battery system, with the exception that the meter coil, A, is 
placed in multiple with the cut-off relay, C. The meter armature 
when attracted is adapted to close the shunt circuit about the meter 
coil and about the cut-off relay coil, this circuit including the re- 
sistance coil, r. 

The cord circuit is the same as that used in the ordinary Western 
Electric system with the exception that a push button key, K y meter 
coil, B, and a lamp, L', have been added to the circuit of the answer- 
ing cord. When the circuit, including the meter coil, B, and the 
lamp, L, in multiple, is closed by pressure of the key, K> in the 
answering cord circuit, a shunt circuit is established around the 
regular supervisory lamp, L, and the usual resistance coil, r' f thus 
allowing a strong current to pass through the third strand of the 
cord to ground through the coil of the cut-off relay, C, and of the 
meter, A. In operation this system is as follows: 

A call is made by a subscriber in the usual manner by the opera- 
tion of the line relay and ^the illumination of the line lamp. In 
answer to such a call the operator will insert the answering plug, 
thus allowing the current to pass through the coil of the cut-off 
relay, C, to ground in the usual manner. A part of this current 
also passes through the magnet, A, of the line meter, but on account 
of the high resistance of this coil this current is not strong enough 
to cause the operation of this meter. The operator then proceeds 
in the usual way to obtain the called subscriber, and if successful 
presses the key, K, belonging to this cord circuit, thus allowing a 
strong current to flow through the third strand of the cord, which, 
in this case, is of sufficient strength to cause the operation of the line 
meter. 

The meter, B, in the cord circuit is common to all the cords of an 
operator's position, and is of very low resistance. The shunt, r, 
around the line meter coil is for the purpose of reducing the resist- 
ance of the circuit through the meter, B, to a sufficient extent to 
allow the operation of this meter and of the lamp, L\ after the line 
meter has been actuated. This meter, B, is for the purpose of as- 
sisting in making peg counts and also in checking the records of 
the line meters. It is obvious that the sum of the meter readings 
of all the cord circuits should agree with the sum of the recorded 
counts on the line meters in a given time. 

The meter used for this purpose is now manufactured by the 



484 



AMERICAN TELEPHONE PRACTICE. 



Western Electric Company as shown in Fig. 352. In this the coil, 
A, is wound on a long core which has a magnetic return through 
the side arms forming the meter frame. The armature is pivoted 
between these arms and has a long stroke. It engages by means 
of the small pawl and ratchet a train of gears belonging to the 
counter. The shunt coil, r, of Fig. 351 is wound on the same spool 
as the meter core, its circuit being completed by the pair of contacts 






FIG. 352.— WESTERN ELECTRIC CONNECTION METER. 



closed by the action of the armature. When the armature is at- 
tracted very little current is required to retain it, and therefore the 
armature does not fall back, even when the shunt circuit is closed 
about it. 

This circuit seems to be extremely objectionable on account of its 
marginal action. It would seem better to use a higher voltage 
source of current than that of the 24-volt battery for operating the 
meter in order to reduce this defect. 



CHAPTER XXVI. 

TOLL SWITCH-BOARD SYSTEMS. 

The methods of handling switch-board connections over long- 
distance lines has undergone a radical change since the advent of 
common battery working, and it is only within the last few years 
that anything like standard practice in this line has been achieved. 
The problems involved are much more complex than those in mak- 
ing connections between the subscribers in a single city, as a greater 
number of operators are necessarily required to make a connection 
than when the connection is of a purely local nature. One great 
factor in efficient toll service is to keep the toll lines themselves 
effectively busy during as great a portion of the time as possible, 
and by effectively busy is meant busy in such a way as to enable 
their use to be charged for. In a toll or long-distance system the, 
greatest investment, as a rule, is in the toll lines themselves, and 
as the revenue for any conversation depends, after a rate between 
points has been fixed, on the length of time the conversation lasts, 
it follows that the greater the use of the lines the larger will be the 
revenue. It is, therefore, of no little importance to so arrange a 
system that as little time as possible shall be lost in making up the 
connection between two subscribers in order to bring them into 
actual conversation, and after the conversation, in taking down the 
connection and leaving the lines free for other use. 

Still another point in the design of toll systems is to so arrange 
the duties of the various operators that the work of the regular 
operators in the city exchanges shall be interfered with as little as 
possible. The work of these operators should be for all prac- 
tical purposes standard, and their signals should be so arranged 
that when a toll connection is called for by a city subscriber, or 
vice versa, their operation will be interpreted by them in the same 
way, as far as possible, as if the connection were of a purely local 
nature. 

As the duration of the connection determines the amount of 
charge for a conversation, it becomes necessary to accurately de- 
termine and record the time which elapsed from the moment the 
two subscribers were actually brought together for conversation 

4S5 



486 AMERICAN TELEPHONE PRACTICE. 

until they finally released the line. This involves a duty not found 
at all in local work. 

For all of these reasons it has become standard practice for the 
local operators, and by local operators is meant the A and B oper- 
ator, in an exchange where trunking is used, to have as little to do 
as possible with the making up of toll connections, this work being 
assigned to special operators skilled in this particular work, and 
who have no responsibility whatever as regards purely local con- 
nections. Very briefly stated, the methods at present employed 
in handling toll connections in connection with large city exchanges 
are as follows: 

A separate toll board is provided which may or may not, accord- 
ing to circumstances, be located in the building in which the city 
exchange is located. Two classes of operators are employed at 
the various positions of the toll board, which may be called "toll- 
line" and "toll-recording" operators. The various toll lines extend- 
ing from the office to the offices of distant cities, and to the vari- 
ous toll stations scattered throughout the country, terminate in this 
toll board in much the same manner as subscribers' lines in the 
magneto multiple switch-board — that is, each line terminates in an 
answering jack and magneto drop in one section, and has a mul- 
tiple jack in each section of the toll board. The answering jacks 
and drops are apportioned among the various operators in accord- 
ance with the amount of traffic on the various lines, the number 
handled by an operator, however, being very much smaller than 
in local work, ranging from 4 to 20 lines in accordance with the 
amount of traffic on these lines. The recording operators occupy 
other positions at the toll board, they being provided with no toll- 
line answering jacks or drops, but usually having in front of them 
multiple jacks of all the toll lines. The purpose of this is to make 
it possible for the recording operator to handle the entire toll busi- 
ness from her position at night. The recording operators' positions 
are provided with trunk lines, terminating in jacks and lamps. 
These trunk lines extend to the multiple switch-board, where they 
appear as multiple jacks in each section. 

It is the duty of the recording operators to receive orders from 
local subscribers for long-distance connections, but they have noth- 
ing to do with calls coming in from toll stations for local subscribers 
or for other toll stations. 

In order to enable connections to be made between toll and local 
subscribers one or more positions are provided on the local mul- 



TOLL SWITCH-BOARD SYSTEMS. 487 

tiple board, in front of which the multiple jacks of subscribers' lines 
appear in the same manner as in front of the regular subscribers' 
positions. These are called "toll-switching" positions, or "toll- 
trunk" positions, and between them and the toll board extend 
trunk lines called toll trunks, by means of which the final connec- 
tion between the local and the long-distance station is actually 
made, regardless of whether the connection originates at the toll 
or local station. These toll trunks are terminated at the toll-switch- 
ing positions in plugs and cords, so that a toll trunk may be con- 
nected with any local line by merely inserting the plug into a mul- 
tiple jack of that line. At the toll board the toll trunks terminate 
in jacks multipled generally once in each section. The arrange- 
ment in this respect is much the same as that between the out- 
going trunk-jacks at the "A" operators' positions and the incoming 
trunk plug at the "B" operators' positions, in purely local work. 
When a local subscriber desires a long-distance connection his call 
is answered by the "A" operator in the ordinary manner. In response 
to his request for long distance, the operator connects his line with 
one of the recording operators' trunks leading to the recording 
operators' positions at the toll board. The "A" operator has noth- 
ing more to do with the connection until the supervisory signals 
tell her by the usual code to pull down the connection. The sub- 
scriber's line is now connected over a recording trunk to one of the 
recording operators' positions, and the recording operator at that 
position has her attention drawn to the fact by the lighting of the 
signal of that trunk line. She therefore plugs into the jack belong- 
ing to that line, and switching her telephone into circuit, obtains 
from the subscriber the number or whatever data is necessarv. of 
the party with whom he wishes to converse, as well as his own num- 
ber and name. It will be seen, therefore, that, aside from the fact 
that it is necessary for her to ring, the local subscribers' operator 
has nothing more to do with the connection than to connect the 
calling subscriber with the recording operator in the same manner 
as that adopted in connecting him with another subscriber in the 
same exchange. 

The recording operator places the required data for connection 
on what is called a "ticket," which she passes to a toll operator. 
who proceeds to find the called-for party if the toll line with which 
the connection is to be made is not in use. Meanwhile, the local sub- 
scriber may be told by the recording operator to hang up his re- 
ceiver and that he will be called, or he may be held waiting, as is 



488 AMERICAN TELEPHONE PRACTICE. 

deemed desirable. In case his line is released, the cord circuit is 
disconnected in the "A" operator's position, the act on the part of 
the subscriber of hanging up his receiver displaying the answer- 
ing supervisory signal, while the act of the recording operator of 
disconnecting from the calling trunk will display the calling super- 
visory signal, the two signals conveying to the "A" operator the" 
order to pull down the connection. 

When the toll operator succeeds in obtaining a connection with 
the called-for party on the toll line she communicates by order-wire 
with the toll-switching operator at the local board and tells her 
the number of the subscriber's line with which connection is to be 
made. The switching operator in return designates the number 
of the trunk to the toll operator and inserts the plug of that trunk 
into the multiple jack of the subscriber who originated the call. 
At the same time the toll operator completes the connection be- 
tween the toll trunk designated and the toll line by means of a pair 
of cords. Some systems are arranged so that the toll operator 
rings the local subscribers, while in others this work falls to the 
lot of the toll-switching operator at the local board. The former 
method is preferable. By means of an automatic time-stamp the 
time when the two subscribers are thus brought together is stamped 
on the toll ticket by the toll operator, and at the end of the conver- 
sation the time is again stamped on the ticket, thus showing accu- 
rately the duration of the conversation. 

When the local subscriber hangs up his receiver at the termina- 
tion of a conversation he either lights a lamp in the toll operator's 
cord circuit or throws the clearing-out drop, depending upon the 
system. The toll subscriber gives the clearing-out signal by turn- 
ing the generator crank in his telephone, thus throwing the drop 
in the cord circuit. No signal whatever is conveyed to the toll 
switching operator until the toll operator pulls down the connection, 
which act lights a disconnect lamp in connection with the toll trunk, 
whereupon the toll trunk operator pulls down the connection and 
leaves the subscriber's line free. 

Connections in the reverse direction, that is, connections wherein 
the call originates on the toll line for a connection with a city sub- 
scriber, are not participated in by the recording operator nor by any 
of the subscribers' operators. Such a call coming over the toll 
line is answered directly by the toll operator, who plugs into the 
line in response to the falling of the line drop. From the informa- 
tion obtained she makes out the ticket and then proceeds to order 



TOLL SWITCH-BOARD SYSTEMS. 489 

up the connection at the toll switching position in exactly the same 
manner as already described. The clearing out is, of course, done 
in the same manner as when the connection originated on the local 
subscriber's line. 

This is a brief outline of the operation of the most modern toll 
system stripped of its details. There are many minor modifications 
of this plan, some of which will be pointed out in considering the 
actual circuits employed in toll work in the remaining portion of 
this chapter. 

To give anything like a comprehensive description of the various 
toll circuits in use in this country as supplied by the various manu- 
facturers, is totally out of the question and would, in fact, be largely 
useless, as the practice among many of the independent manufactur- 
ers is changing so rapidly as to call for a new system in every in- 
stallation. The practice of the Bell companies may be considered 
as representative of the best thought on this subject, and from 
among the various systems installed by these companies during the 
past ten years, three stand out conspicuously. These three, which 
the writer has, for the sake of convenience, arbitrarily referred to 
as systems A, B and C, will therefore be described as typical of the 
development of toll system circuits since common battery work 
became well established. 

System "A" differs from system "B" mainly in that only two wires 
are required in a toll trunk extending between the toll-switching 
operator's positions at the local multiple switch-board and the toll 
board. In system "B" three wires are employed in each of these 
toll trunks. In systems where the local board and the toll board 
are in the same building, it makes very little difference whether two 
or three wires are required in the trunks between the toll and local 
boards, but where these boards are widely separated, as is often the 
■case, the toll board being in some outlying part of town in a posi- 
tion where the toll lines may be brought to it with a minimum use 
of underground cable, the requirement of three wires extending 
from the toll board to the local board would be a serious draw- 
back. 

System "A" requiring only two wires, was therefore designed for 
relieving this difficulty, while system 4 'B," requiring three wires for 
the trunk, has been used only where the two boards were located in 
the same building or very close together. The circuit of the toll 
trunk line extending between the toll-switching operator at the 
multiple board and the various multiple jacks on the toll board is 



490 



AMERICAN TELEPHONE PRACTICE. 



shown in Fig. 353. This circuit, it will be seen, terminates at the 
toll-switching operator's position in a cord and plug, being provided 
with the usual ringing keys which may be of any ordinary type for 
either manual or automatic ringing, according to the necessity of 
the case. This plug and cord circuit terminates in one half of a re- 
peating coil, between the center points of which is bridged the bat- 
tery for feeding current to the local subscribers' lines. The super- 
visory relay, A, is included in the ring strand of the cord, the func- 
tion of this relay differing, however, from that of the usual cord 
circuit supervisory relay, as will be pointed out. 




SWITCHING OPRS CORD CIRCUIT 



toll opns.poa. 



FIG. 353.— WESTERN ELECTRIC TOLL TRUNK LINE, SYSTEM A. 



The two wires of the trunk line extending between the common 
battery office terminate in the second half of. the repeating coil at 
the toll-switching operator's position. Between the center points 
of this half of the repeating coil is bridged a condenser which, how- 
ever, under certain conditions, is adapted to be shunted by the action 
of several of the relays, as will be described. At the toll switch- 
board the trunk line passes through a multiple jack at each of the 
toll sections, but ordinarily no such jack is provided for the trunk 
line at the recording operator's positions. These jacks are provided 
with test rings which are connected to ground through the cut-off 
relay, B, in the same manner as the test rings of the jacks on the 
local line. When, therefore, a connection is made with a trunk 
line at one of the toll sections, this relay B is operated by the bat- 



TOLL SWITCH-BOARD SYSTEMS. 491 

tery of the cord circuit and remains operated as long as the plug is 
so inserted in the jack. The test rings of the toll trunk line are 
thus made to test busy during such a connection in the same way 
as in a local line. The various relays and other apparatus asso- 
ciated with the two ends of the toll trunk line will be more fully 
described in connection with Fig. 354, which shows a local sub- 
scriber's line connected through the toll-switching operator's posi- 
tion, the toll trunk extending to the toll board, thence through the 
toll operator's cord circuit to the toll line which is shown at the 
right-hand end of the figure. Assuming that the local subscriber 
originates a call and desires a toll connection, this call will be an- 
swered by his "A" operator in the usual manner. Learning that a 
toll connection is desired, the subscriber's operator will press the 
button of an order-wire leading to the toll-switching operator's 
position and will order up the toll trunk connection specifying the 
number of the subscriber who made the call. Upon receiving this 
order the toll-switching operator will take up the plug of an idle 
toll trunk and insert it into the multiple jack of the called sub- 
scriber, disregarding the fact that this line tests busy on account 
of the presence of the "A" operator's answering plug in the answer- 
ing jack of the line. The "A" operator, after this connection is 
made, will withdraw the answering plug from the jack and will have 
nothing more to do with the connection. The insertion of the toll 
trunk plug into the multiple jack of the subscriber's line will con- 
nect that subscriber's line through to the toll board, and since the 
calling subscriber still has his receiver off its hook, will cause the 
lighting of the lamp, L, having a white cap, at the recording 
operator's position. The means by which this is brought about 
may be described as follows: The circuit of the white lamp, L, at the 
recording operator's position is controlled at the back contact of 
two relays, C and D. The relay C is normally de-energized and 
therefore keeps its contact in the circuit of the white lamp closed. 
The relay D is, however, normally energized and therefore keeps 
the circuit of the white lamp open. The circuit over which the relay 
D is normally energized may be traced from ground at the 
recording operator's position through the coil of this relay, thence 
from the back contact of the relay B over one side of the trunk line 
to the toll-switching operator's position at the multiple board, and 
thence through one winding of the repeating coil through the back 
contact of the relay E, to the non-grounded side of the battery. 
This flow of current continues at all times except when the trunk 




^/y^|i|i|i|i|i|4M(WW 







FIG. 354, 



-COMPLETE WESTERN ELECTRIC TOLL CIRCUIT, SYSTEM A. 
492 



TOLL SWITCH-BOARD SYSTEMS. 493 

line is in use and therefore the energization of the relay D nor- 
mally holds the circuit of the white lamp open. When the toll- 
switching operator plugged into the multiple jack of the subscriber's 
line the flow of current took place through the relay A over the 
metallic circuit of the subscriber's line, since the calling subscriber 
had its receiver off its hook and thus caused this relay to attract its 
armature, thus closing the circuit through the relay F and the relay 
E from ground to battery. The operation of the relay E, which was 
thus caused, opened the normally closed circuit of the relay D, 
allowing its armature to drop back and thus lighted the white lamp. 
Upon seeing the white lamp lighted the recording operator will 
connect her telephone across the circuit of the trunk line by means 
of her listening key, K, and thus be enabled to converse with the 
calling subscriber over the combined circuit of the toll trunk line 
and the subscriber's line. From the information thus obtained the 
operator makes out the ticket, giving the proper data for complet- 
ing the connection and passes it to a toll-line operator. The act of 
listening in on the trunk line on the part of the recording operator 
will cause the operation of the relay C, through the back contact of 
which the circuit of the white lamp extends. The operation of this 
relay therefore extinguishes the white lamp but lights in its place a 
green one, L, the circuit of which also passes through the back con- 
tact of the relay D. When the recording operator restores her 
listening key she will not extinguish the green lamp because the 
relay C is self-locking, it carrying an additional armature which, 
when closed, completes the circuit from the coil of the relay through 
the back contact of the relay B. The purpose of furnishing two 
lamps at the recording operator's position is to make it impossible 
for the recording operator, after she has answered the call and before 
the toll operator has taken up the connection, to forget that she 
has already responded to the lighting of the white lamp and again 
speak to the subscriber. If one lamp only were furnished, it would 
necessarily have to be arranged so that it would relight when the 
recording operator threw her listening key back into its normal 
position. Otherwise there would be danger Fhat the trunk and the 
subscriber's line would be "held up." 

The toll operator having received the ticket from the recording 
operator will insert the answering plug of the toll-cord circuit into 
the multiple jack of the toll trunk line. This will extinguish the 
green lamp at the recording operator's position by causing the 
operation of the relay B. The toll operator will then insert the 



494 AMERICAN TELEPHONE PRACTICE. 

calling plug of the cord circuit used into the multiple jack of the 
desired toll line, and after ringing on that line and securing the 
subscriber, bring the two subscribers together for conversation. 
When the toll-switching operator inserted the toll-trunk plug into 
the jack of the calling subscriber the relay G was caused to oper- 
ate over a circuit extending from ground through the winding of 
the cut-off relay, the coil of the relay G, and the ringing lamp to 
battery and ground. This would cause the operation of the ring- 
ing lamp were it not for the fact that the calling subscriber has his 
receiver off its hook, and therefore the operation of the relay A 
had caused the operation of the relay F, which, when operated, 
had locked by virtue of its 40-ohm winding. This 40-ohm coil 
is, therefore, present as a shunt about the ringing lamp and pre- 
vents its lighting. 

At the end of the conversation the clearing-out signal will be 
received at the toll board by the falling of the drop in the cord 
circuit. This drop may be made to fall either by generator cur- 
rent coming in over the toll line in an obvious manner, or by the 
hanging up of the receiver of the local subscriber. The means by 
which the hanging up of the calling subscriber's receiver causes the 
falling of the clearing-out drop bridged across the toll operator's 
cord is rather obscure, but it may be traced as follows: 

The hanging up of the called subscriber's receiver opens the me- 
tallic circuit of the line, thus de-energizing the relay A, allowing 
the relay E to become de-energized. The falling back of the two 
armatures of this relay closes a circuit at two points, which may 
be traced from ground through one back contact of relay E and the 
winding of the relay H, one winding of the repeating coil, over 
the tip side of the trunk to the tip side of the multiple jacks in the 
toll sections, thence to the toll operators' cord and through the 
clearing-out drop to the sleeve side of the multiple jacks, thence 
back over the sleeve side of the trunk through a second winding 
of the repeating coil, through the second back contact of the re- 
lay E to battery. Receiving this signal, the toll operator will pull 
down the connection, and in removing the plug from the toll-trunk 
jack she will light the disconnect lamp of the toll-switching oper- 
ator at the multiple board. This will be accomplished by the fol- 
lowing means: The relay H, it will be remembered, was operated 
when the local subscriber hung up his receiver, owing to the de- 
energization of the relay E. When the relay H was energized 
it caused the energization of the relay /, also the circuit through 



TOLL SWITCH-BOARD SYSTEMS. 495 

this latter relay being from ground at the relay H, through the 
front contact of that relay, thence through the 40-ohm coil and the 
winding of the relay /, through the front contact of the relay G, 
to the live side of battery. The pulling up of the armature of the 
relay / causes that relay to lock, since the path was thus estab- 
lished from the live side of the battery through the front contact 
of the relay G, the coil of the relay /, and from the front contact 
of this latter relay through the disconnect lamp to ground. This 
disconnect lamp was not, however, operated by the current which 
thus flowed through it because of the presence of the 40-ohm shunt 
to ground between the contacts of the relays H and /, both of which 
relays are now operated. When, however, the toll operator with- 
drew the plug from the multiple jack of the toll-trunk line the re- 
lay H was de-energized, thus breaking the 40-ohm shunt around 
the disconnect lamp and causing the illumination of that lamp. 
Upon seeing the disconnect lamp lighted, the toll-switching operator 
will pull down the connection, thus restoring all apparatus to its 
normal position. 

When a toll subscriber desires to converse with a multiple board 
subscriber the toll line operator will be signaled in the regular man- 
ner by the falling of the toll-line drop at the toll board. The toll 
operator will then insert the answering plug of a pair of toll cords 
into the toll-line jack, and after throwing the listening key and find- 
ing out the number of the common battery line with which the 
connection is to be established, speak over an order-wire to the 
toll-switching operator, telling her that a toll connection is to be 
established with a certain common battery line. The toll-switch- 
ing operator will designate a trunk for use, and with the plug of 
this trunk test the multiple jack of the local subscriber's line. If 
the line is busy she will insert the trunk plug into the busy jack 
(not shown in figure), which will notify the toll operator by "tone" 
that the line is busy. In case, however, the line is free she will 
insert the trunk plug into the multiple jack, whereupon the ring- 
ing lamp associated with the trunk plug at the toll-switching oper- 
ator's position will light immediately, and the toll-trunk operator 
will ring the subscriber. The illumination of the ringing lamp will 
be brought about by current flowing from the live side of battery 
through this lamp, the coil of relay G to the sleeve contact of the 
jack, thence to ground through the cut-off relay of the subscriber's 
line. When the subscriber answers the call the ringing lamp goes 
-out and cannot be relighted until the trunk plug has been with- 



496 AMERICAN TELEPHONE PRACTICE. 

drawn from the jack, this state of affairs being brought about pri- 
marily by the operation of the relay A, due to the flow of current 
from the subscriber's telephone. The operation of this relay will 
cause the operation of the relays F and E, the closing of the contact 
of the relay F serving to close the circuit through the locking 
coil of this relay, which coil forms a 40-ohm shunt about the ring- 
ing lamp, thus extinguishing it. This locking coil remains effect- 
ive in holding up the armature of the relay F until the connection 
is finally broken by the withdrawal of the trunk plug from the mul- 
tiple jack. 

At practically the same time that the toll-switching operator 
inserted the trunk plug into the jack of the subscriber's line the 
toll operator completed the connection between the toll line and 
the multiple jack of the toll trunk, thus bringing the subscribers 
into talking relation, the connection between the two subscribers 
being exactly the same as that shown in Fig. 354, except that the 
answering plug of the toll operator's cord circuit is now inserted 
in the toll-line jack instead of in the toll-trunk jack. The toll oper- 
ator's cord circuit used in this system is adaptable without change 
to use in making connections between toll subscribers or between 
toll and local subscribers. 

The clearing-out drop used in the toll operator's cofd circuit is, 
as will be seen from the diagram an electrically-restoring drop of 
the type shown in Fig. 178. This is restored whenever the toll 
operator throws the listening key. This is accomplished by an 
extra pair of contacts which close when the key is operated and 
completes the circuit from the live side of battery through the re- 
storing coil of the drop. Frequently, also, the coil of this drop is 
wired through an additional key so that it may be cut out of circuit 
in the case of a very long distance connection over which talking 
is difficult, in which case the bridge afforded by the drop would, 
if left permanent, effect to a slight extent the talking efficiency. 

The circuits of the American Bell Telephone Company's sys- 
tem "B" complete, from a local subscriber's line on the left to a 
toll line on the right, are shown in Fig. 355. 

In this system the repeating coil which separates the common 
battery line from the toll line is included in the cord circuit itself 
instead of in the trunk line. One-half of each cord circuit, in this 
case, at the right of the cut, is wired the same as half of a cord 
circuit at the local multiple board, the other half being wired sub- 
stantially, as in ordinary magneto practice, with a self-restoring, 




#3fllp*fe 




TOl_l_ LINE. 




FIG. 355.— COMPLETE WESTERN ELECTRIC TOLL CIRCUIT. SYSTEM B 
32 407 



498 AMERICAN TELEPHONE PRACTICE. 

clearing-out drop bridged across the tip and ring strands. Each 
cord circuit is equipped with a repeating coil key, K 3 , which is so 
wired that when thrown the repeating coil, K, is cut out of circuit 
and a straight circuit obtained between the two plugs, so that the 
same cord circuit may be used for establishing toll-to-toll connec- 
nections as is used for establishing connections between a toll and 
a local line, the only difference being in the position of the repeat- 
ing coil key at the time of use. When key, K 3 , is thrown in this 
manner a 210-ohm resistance coil is thrown across the common 
battery part of the cord circuit, and the 40-ohm shunt is placed 
around the supervisory lamp. 

In this system a disconnect lamp appears at the toll-switching 
operator's position at the local multiple board, but no ringing lamp. 
The absence of a repeating coil in the toll-trunk circuit makes it 
possible for the toll-line operator to ring the bell of the local sub- 
scriber instead of having the toll-trunk operator do this, as in sys- 
tem "A." This is, of course, an advantage, as it enables the oper- 
ator to be the true master of the situation and to more completely 
supervise the call than in cases where the toll-trunk operator is 
compelled to do the ringing. As will be noticed, the toll-trunk 
circuit, extending between the toll-switching position at the local 
board and the multiple toll board, contains three wires, thus prac- 
tically limiting this system to use in exchanges where the toll and 
multiple boards are in the same building. 

The operation of establishing a connection when a call is orig- 
inated by a local subscriber is practically the same as far as the 
work of the operators is concerned, as that described in system A. 
The operation of the apparatus required to bring about the same 
signaling results is, however, somewhat different, and is as follows: 
The "A" operator upon receiving a call for long distance will com- 
municate by order-wire to the toll-switching operator who will, as 
in svstem "A," insert the toll trunk plug into the multiple jack of 
line designated. This will cause the operation of the relay G which 
will cut off the test circuit and complete the connection between 
the tip strand of the trunk plug and the tip side of the trunk line. 
The operation of this relay will also connect a ground to one side of 
the disconnect lamp, which lamp will not, however, be operated be- 
cause of the inactivity of the relay F. No change on the aspect 
of the signals will therefore take place at the switching operator's 
position. Since, however, the calling subscriber has his receiver 
off its hook, the double-wound relay A will be energized over a 



TOLL SWITCH-BOARD SYSTEMS. 499 

path which may be traced from the live side of the battery through 
the back contact of one lever of the relay B, through the coil of the 
relay A, to the ring side of the trunk, thence out over the ring side 
of the subscriber's line and back to the tip side of the line and trunk, 
through the other coil of the relay A, and the second back contact 
of the relay B, to earth. It will be seen that when the relay B 
operates it opens both windings of the relay A, thus rendering it 
inert. The operation of the relay A, when the toll-switching opera- 
tor made the connection with the subscriber's line, illuminated a 
lamp, L, with a white cap at the recording operator's position over 
a circuit which may be traced from ground through the contact of 
the relay A, to the back contact of the relay C, the lamp and the 
pilot relay to the' live side of the battery. The illumination of the 
white lamp is a signal for the recording operator to come in on the 
circuit, which she does by throwing a listening key, K lf thus con- 
necting her telephone with the circuit and enabling her to com- 
municate with the subscriber. From the information thus obtained 
she makes out the ticket and passes it to a toll operator. The extra 
pair of contacts carried on her listening key closed the circuit ener- 
gizing relay, C, which carries a contact which closes a locking 
circuit for this relay. The relay C carries an additional lever 
which, when operated, switches the circuit from the white lamp to 
a green one, L, so that when the recording operator listens in the 
white lamp goes out while the green lamp is lighted. The 300-ohn? 
shunt around the white lamp is provided so that in case this lamp 
burns out the pilot will light and call the operator's attention. When 
'the toll operator receives the ticket she inserts the toll plug of a 
cord circuit into the multiple jack of the toll trunk line. This will 
extinguish the green lamp at the recording operator's position be- 
cause the relay B will be operated over the third strand of the toll 
operator's cord and the sleeve of the toll trunk circuit. The opera- 
tion of this relay, as before stated, opens both sides of the circuit 
through the windings of the relay A, which, when de-energized, 
breaks the circuit through the green lamp at the recording opera- 
tor's position, thus putting it out. The relay C which was operated 
when the recording operator listened in, and locked in that posi- 
tion, is, at the same time, de-energized because short-circuited, this 
short circuit existing between the winding of the relay and ground 
through the front coxitact of the left-hand lever of the relay B. 
All apparatus at the recording trunk operator's position, except 
the relay B, is thus restored to its normal condition, and this relay. 



500 AMERICAN TELEPHONE PRACTICE. 

B, which will be energized as long as the connection exists with the 
trunk line at the toll board, will thus prevent the subsequent oper- 
ation of any apparatus at the recording operator's position. 

The operation of the relay B has another function in that when 
operated it holds the disconnect lamp at the toll-switching oper- 
ator's position in such position as to enable this lamp to be oper- 
ated when further changes are made. The operation of the relay 
B closes the circuit from battery through the third strand of the 
trunk line, the 40-ohm coil of the relay F, and thence to ground 
through the contact of the relay G. The relay F is thus energized 
and the contact it closes serves to close a circuit through a locking 
coil on this relay and the disconnect lamp. The relay F is, there- 
fore, held energized until its locking circuit is finally broken by the 
de-energization of the relay G, which takes place when the final 
disconnection is made. Although this locking current passes 
through the disconnect lamp, this lamp is not illuminated because 
of the presence of the 40-ohm shunt about it, this shunt being the 
40-ohm coil of the relay F. The toll operator having inserted the 
local plug into the multiple jack of the trunk line designated, will 
proceed to call the toll station by inserting the corresponding toll 
plug into the jack of the line and ringing. 

The connection is now established between the toll line and the 
subscriber's line, all signals at all positions being de-energized. 
It will be noticed that the supervisory signal on the common bat- 
tery side of the toll operator's cord circuit will be placed under the 
control of the common battery subscriber in the same manner as 
if that cord were connected directly to the common battery sub- 
scriber's line. At the end of the conversation a double clearing- 
out signal will be received at the toll board due to the falling of 
the drop on the toll side of the cord and the lighting of the super- 
visory lamp on the common battery side. The toll operator will 
then throw her listening key, thus restoring the clearing-out drop, 
and listen in to satisfy herself that a second connection is not de- 
sired. If no connection is desired, the operator will withdraw the 
plugs from the toll trunk and toll-line jacks. The act of withdraw- 
ing the plug from the toll-trunk jack will de-energize the relay B, 
which will open the circuit through the 40-ohm coil of the relay F, 
at the switching operator's position, thus removing the 40-ohm 
shunt about the disconnect lamp and causing its illumination. This 
is the signal for the toll-switching operator to pull down the con- 
nection; which act will restore all apparatus to its normal condition. 



TOLL SWITCH-BOARD SYSTEMS. 501 

A toll-to-local connection is established in practically the same 
way as in system "A." As in that system, the "A" operator and the 
recording operator play no part in the connection. The means by 
which the signals are made to operate, however, is different, al- 
though practically the same results are produced. The toll sub- 
scriber signals the toll board in the usual manner, and the oper- 
ator inserts the plug belonging to the toll side of one of her cord 
circuits into the answering jack of the calling line. After finding 
out that the call is for a local subscriber, and making out the ticket, 
she will order up the connection at the toll-switching position at 
the multiple board by the order-wire, and the toll-switching oper- 
ator will test the line in the usual manner. If she finds it free she 
will insert the toll-trunk plug into the multiple jack of the local 
subscriber called for, and thus complete the connection between the 
local subscriber and the toll operator, who by this time has inserted 
the plug on the common battery side of her cord circuit into the 
toll-trunk jack designated by the toll-switching operator. As the 
called-for subscriber has his receiver on its hook, the supervisory 
lamp in the common battery side of the toll operator's cord cir- 
cuit will be lighted. Thus this lamp serves as a ringing signal for 
this operator who, by means of the ringing key, will call the local 
subscriber. When the subscriber responds, the supervisory signal 
will go out and the two subscribers will be in position for conver- 
sation. 

The insertion of the toll operator's plug into the toll-trunk jack 
operated the relay B, at the recording operator's position. No 
effect is produced on the signals at the recording operator's posi- 
tion, as, due to the fact that the relay B is energized before the 
subscriber answers, relay A is not operated. The operation of the 
relay B placed the disconnect lamp at the switching operator's 
position in proper position to be operated when the relay B is 
again de-energized, as above pointed out. The disconnect signals 
are received in the same manner as when the call originated at the 
local subscriber's end. 

The method of making connections between the toll subscribers 
in these two systems, "A" and "B," is almost identical. In both 
cases a toll subscriber signals the toll board by turning his gene- 
rator crank and thus throwing the line drop at the toll board. The 
operator will insert an answering plug, and after throwing her 
listening key speak to the subscriber and find out that a connection 
is desired with some other toll station. In system "A" the eord 



502 AMERICAN TELEPHONE PRACTICE. 

circuit can be used for making either a toll-to-toll or toll-to-local 
connection without change, but in system "B" the cord circuits are 
normally suited for making connections between toll and local sub- 
scribers. On this account it is necessary for the toll operator in 
system "B" to throw a key associated w T ith the cord circuit before 
this cord circuit can be used for connecting the toll subscribers. 
Having done this, the remainder of the operation is the same as in 
system "A." After testing, the operator will insert the calling plug 
associated with the answering plug in use into the multiple jack of 
the desired line, and ring. Until the subscriber answers, it is neces- 
sary for the operator to listen in at short intervals so as to know 
when the call is answered. The operator who has charge of the 
calling toll line will make out a ticket and record on it the duration 
of the conversation, the number of the calling and the number of 
the called toll stations, and the names of the calling and called 
parties. 

Neither system "A" nor "B" seems to have been considered satis- 
factory by the engineers of the Bell telephone interests, and there- 
fore what the writer has called system "C" was devised. This sys- 
tem embodies the advantages of both systems "A" and "B," with 
as few as possible of their disadvantages. The toll-trunk circuit 
requires but two wires, thus making the system adaptable to cases 
in which the toll board is in the same building with the local board, 
or at some distant point. The feature of having the toll operator 
ring the local subscriber in a toll-to-local connection is obtained, 
although a repeating coil is associated with the trunk line instead 
of with the toll operator's cord circuit. 

The advantage of system "A" over system "B" — that the toll 
operator's cords are universally adaptable without change to either 
local-toll or toll-to-local or through connections — is retained, as is 
also the advantage of a two-wire trunk circuit, but it must be stated 
that these various desirable features have been attained at the ex- 
pense of great complexity in the toll-trunk circuit, this circuit hav- 
ing no less than seven relays. Several features of advantage not 
found in either systems "A" or "B" are present, the general method 
of operation being quite different. In this system, when a local 
subscriber calls for a long-distance connection, the "A" operator 
who answers the call, instead of ordering up the connection at the 
toll-switching operator's positon, as in systems "A" and "B," sim- 
ply completes the connection between the calling line and a record- 
ing operator's trunk by means of a special pair of cords. Aside 



TOLL SWITCH-BOARD SYSTEMS. 503 

from the fact that she does not ring in this case, she makes this 
connection and obeys the subsequent supervisory signals in the 
cord circuit in exactly the same manner as if the connection were 
between two local subscribers. The recording toll operator com- 
municates with the subscriber over this trunk and then either orders 
up the connection herself at the toll-switching position, or allows 
the toll operator to do this after receiving the ticket, as will be de- 
scribed. Upon receipt of a call for long distance, the "A" oper- 
ator will insert the answering plug of a special pair of cords, which 
are termed "tone-test" cords, into the multiple jack of the calling 
line, withdrawing the answering plug with which she answered the 
call from the answering jack of that line. With each plug of the 
"tone-test" circuit is associated a lamp, the plug adapted to con- 
nection with the jacks of the common battery line being associated 
with one having a white cap, and the lamp associated with the other 
plug, which is adapted to connection with the recording toll trunk, 
having a red cap. The white lamp in the tone-test cord circuit 
will remain unlighted when the answering cord is thus connected 
with the calling subscriber/s line, since this subscriber has his re- 
ceiver off its hook. With the remaining plug of the pair the "A" 
operator will test the multiple jacks of the recording toll trunk 
lines which terminate in multiple jacks on the local multiple board, 
and upon finding one which is not busy, complete the connection 
by inserting this plug into it. The red lamp associated with this 
plug will also remain unlighted, but a lamp at the recording oper- 
ator's position associated with the particular recording toll trunk 
used will light, and the recording operator will plug into the corre- 
sponding jack of this trunk, thus connecting her telephone with 
the calling subscriber, from whom she will take the necessary in- 
formation for making out the ticket. The cord circuit, by which 
the recording operator is enabled to communicate with the local 
subscriber, is provided with two lamps and terminates in a single 
plug. One of these lamps, the holding lamp, is placed under the 
control of the calling subscriber when the plug with which it is 
associated is connected through the toll trunk with the line of that 
subscriber. The second lamp is a disconnect lamp. The operation 
of both of these lamps will be described further on. After making 
out the ticket the recording operator has two courses of action 
opened to her, her decision between them depending on whether 
or not she has reason to believe that the connection called for may 
at once be obtained. If she thinks it will be some time before it 



504 AMERICAN TELEPHONE PRACTICE. 

will be possible to establish the connection desired, as in case the 
subscriber must be hunted up in some large and distant city, she 
will tell the calling subscriber to hang up his receiver and wait until 
called. The hanging up of the subscriber's receiver will light the 
white lamp of the tone-test cord used in making the connection, 
but the subscriber's operator will pay no attention to this, her sig- 
nal for taking down the connection being the lighting of the red 
lamp. Having told the calling subscriber to hang up his receiver, 
the recording operator will first pass the ticket to the toll-line oper- 
ator and remove her plug from the recording toll-trunk jack, which 
act will light a red lamp in the tone-test cord at the "A" operator's 
position. The lighting of this lamp gives a signal to the subscrib- 
er's operator to take down the connection, and she, therefore, re- 
moves both plugs, leaving the subscriber's line free for other busi- 
ness. The recording operator has now nothing further to do with 
the connection, the ticket being in the hands of the toll operator, 
who supervises all further work in connection with it. 

If, after making out the ticket, the recording operator had reason 
to believe that the connection could be at once obtained, she would, 
by means of an order-wire key, speak directly to the toll-switching 
operator at the multiple switchboard and give her the number of 
the line with which the connection is to be made. In response 
to this the toll-switching operator will designate to the recording 
operator the number of the trunk to be used, which number the 
recording operator will place on the ticket, together with the other 
information. The toll-switching operator will then test the mul- 
tiple jack of the subscriber's line and receive the special busy test 
due to the presence of the tone-test plug in the multiple jack at 
the section in which the call originated. She disregards this sig- 
nal and plugs into the jack with the plug of the toll trunk desig- 
nated to the recording operator. 

After the toll-switching operator has inserted the plug of the 
designated trunk into the multiple jack of the calling line the re- 
cording operator will withdraw her plug from the recording trunk- 
jack, thus giving a signal to the "A" operator to disconnect with 
the tone-test plugs, and insert it into the multiple jack of the toll- 
trunk line. These trunk lines are multipled in all toll and record- 
ing sections. This enables the toll-recording operator to com- 
municate again with the subscriber over a different route, and con- 
vince herself that the connection so far is complete and that the 
subscriber has given his proper number. She will then pass the 



TOLL SWITCH-BOARD SYSTEMS. 505 

ticket to the toll operator who has charge of the toll line with which 
connection is to be established. The calling subscriber cannot now 
signal the "A" operator, who has removed the tone-test cord from 
his jack, but he is enabled to signal the recording operator by means 
of the holding lamp in that operator's circuit, the path over which 
this signal is thus communicated to the recording operator being 
through the multiple jack of the subscriber's line at the toll-switch- 
ing position, and thence over the toll trunk to the recording oper- 
ator's position. 

We have now traced the operation of this system to a point where 
the toll operator takes up the work, she having received a ticket 
which in one case was provided with information as to what trunk 
should be used, in which case she would know that the connection 
was already ordered up at the toll-switching position, and that she 
had only to connect that trunk with the proper toll line. In the 
other case, the ticket not being provided with the number of the 
trunk, she would know that the calling subscriber was not waiting 
and that the toll trunk had not been ordered up. 

Taking the case where no trunk is designated, the toll operator 
will first insert the calling plug into the multiple jack of the desired 
toll line and ring in order to secure the party desired. After she 
has secured this party she will give the switching operator, over 
an order-wire, the number of the line of the subscriber who made 
the call, and will receive in return the designation of the trunk, 
whereupon she will insert the answering plug into the multiple 
jack of the designated line and call the desired subscriber by ring- 
ing, the toll-switching operator having made the connection be- 
tween the multiple board end of the trunk and the subscriber's line. 

If, on the other hand, the toll operator finds that the trunk line 
is designated on the ticket, she will know that the recording oper- 
ator has already ordered up the connection, and that the subscriber 
is waiting. Under this circumstance she will first insert the an- 
swering plug into the trunk of the jack designated on the toll ticket. 
This act will light the disconnect lamp in the cord circuit of the 
recording operator. The lighting of this lamp will be the signal 
for the recording operator to pull down the connection at her posi- 
tion. The toll operator will then make the connection between the 
jack of the trunk line used and the toll line and call the toll sta- 
tion by ringing. Connection is now established between the toll 
station and the common battery subscriber. The wiring of all the 
circuits over which conversation takes place is shown in Fig. 356. 



M-VOLT5 ^ <■ 




^=^^3 



FIG. 356.-COMPLETE WESTERN ELECTRIC TOLL CIRCUIT, SYSTEM C. 

506 



TOLL SWITCH-BOARD SYSTEMS. 507 

The toll-trunk circuit shown in this figure shows it in the form 
which was patented, but differs in one or two respects from the cir- 
cuit which the Bell companies are now using. Relay E has been 
replaced by one of a different type, but the method of operation, 
as described above, has not been changed. 

When the trunk plug was inserted by the toll-switching operator 
into the multiple jack of the calling subscriber's line in response 
to the order received from the toll-recording operator or from the 
toll operator, the relay A at the toll switching position was ener- 
gized. This severed the normally completed test circuit belonging 
to the trunk plug and completed the circuit from the tip of the plug 
to the tip side of the trunk line. The operation of this relay also 
closed a circuit from the grounded side of battery at the toll board 
through the upper contact of relay H over the trunk line to the 
common battery exchange, then through one winding of the re- 
peating coil, the back upper contact of relay F, winding of relay B, 
the contact of relay A, the second back contact of relay F, a second 
winding of the repeating coil, thence back over the trunk line to 
the toll board through the second back contact of relay H, and 
resistance, r 2 , to the non-grounded side of battery. 

Current flowing over the circuit will at once operate relay B and 
illuminate the disconnect lamp, L, in the toll trunk circuit. How- 
ever, as soon as the toll operator inserted the toll plug into the toll 
trunk jack (this would happen at practically the same instant in 
which the switching operator inserted the trunk plug into the mul- 
tiple jack and hence this lamp in actual practice would seldom do 
more than flash), relay H was operated and the circuit above 
traced out opened in two points at this relay, thus extinguishing 
the disconnect lamp and removing the battery connections from the 
toll side of the trunk line. The method by which the toll operator 
is able, by throwing her ringing key, to ring the bell of the common 
battery subscriber is as follows: When the ringing key is thrown, 
current from the ringing generator will pass out over the ring side 
of the trunk line, one winding of the repeating coil, one back con- 
tact of relay F, through condenser, C, the winding of relay D, and 
back through the second set of springs of relay F, the second wind- 
ing of the repeating coil, the tip side of the trunk line to the other 
side of the generator. It will be noticed that relay B acts as a 
shunt to the condenser and relay D, but as relay B is of high re- 
sistance, it will have no appreciable effect upon the operation of 
relay D. Relay D will then be energized and de-energized very 



508 AMERICAN TELEPHONE PRACTICE. 

rapidly and battery current will flow from ground through its arma- 
ture, the winding of relay C, and resistance, r 3 , to the non-grounded 
side of the battery. This will operate relay C, and thus close a cir- 
cuit for ringing current from the generator at the common battery 
board out over the subscriber's line and through the bell. When 
the subscriber answers, relay G will be operated and battery current, 
will then flow from ground through the contact of this relay, wind- 
ing a, of relay E and resistance, r 3 , to the non-grounded side of 
battery, thus energizing relay E It will be noticed that when this 
relay, E, is operated, there will be a circuit from ground through the 
winding of relay F, contact of relay E, both windings of relay E, 
and resistance, r 3 , to battery. However, as long as relay G is 
operated, relay F and winding, b, of relay E will be short-circuited 
and hence no current will pass through either of these windings. 
Relay £ is a slow-acting differential relay arranged mechanically 
so that a small period of time will elapse after current has been with- 
drawn from the winding before it will release its armature. When 
current flows through both of these windings in series, the mag- 
netism set up by these two windings will neutralize each other and 
release the armature. Hence, at the end of conversation, when the 
common battery subscriber hangs up his receiver, thus de-energizing 
relay G, the short circuit which has been placed around relay F 
and winding b of relay E will be removed and current will pass 
through both windings of relay E and the winding of relay F . 
Relay F will be energized immediately, before the differential 
action of relay E gets in its work, so that, for an instant, or before 
relay E is demagnetized, relay F will pull up its armature, thus 
closing a circuit from ground back over the trunk line through 
the clearing-out drop in the toll operator's cord circuit at the toll 
board, back on the other side of the trunk line to the negative or 
non-grounded side of battery at relay F. Thus, the clearing-out 
drop will fall and give a signal to the toll operator to disconnect. 
When the clearing-out drop has fallen and before the toll operator 
has withdrawn the plug, all of the relays in the multiple-board end of 
the toll trunk circuits, excepting relay A, are in their normal posi- 
tion. When the toll operator withdraws her plug, she will de- 
energize relay H and allow battery to pass over the trunk line and 
through winding of relay B, thus lighting the disconnect lamp in 
front of the switching operator. The toll-switching operator will 
then take down the connection. 

When a call is received over a toll line at the toll board and the 



TOLL SWITCH-BOARD SYSTEMS. 



509 



toll operator upon answering finds that a local connection is desired, 
she will speak over an order-wire to the switching operator at the 
multiple board. The switching operator will designate the trunk 
and insert the plug of this trunk into the multiple jack of the desired 
local line. The toll operator at the same time will insert the calling 
plug into a jack of the trunk line and ring. , Clearing-out signals 
are in this case received in exactly the same way as when the con- 
nection is made from local-to-toll. 

The methods which the Bell Telephone companies use for 




FIG. 357-SECTION OF TOLL BOARD. 



handling connections between toll stations are numerous, but in 
many cases one or more separate positions are provided, called 
"through positions." 

In a case having these through positions, if the toll operator re- 
ceives a call from a toll line for another toll line, she will make out 
a ticket for it in the same manner as if the call were for a local line 
and, on completing the ticket, will establish the connection herself 
if it is possible to do so at once. If, however, the called for line is 
in use, she will pass the ticket to a through operator who has in 
front of her a multiple jack of each line, and in some exchanges a 



510 AMERICAN TELEPHONE PRACTICE. 

multiple lamp which remains lighted as long as that line is busy. 
This operator, as soon as she finds out that both lines are free, either 
by noting that the lamps of both are extinguished, if her position is 
provided with lamps, or by testing, in case she has none, will estab- 
lish the connection. At the end of the conversation on such a con- 
nection the clearing-out drop will fall in the ordinary manner when 
either of the toll subscribers turns his generator crank. 

The tendency among Bell companies seems to be to use lamps 
and locking relays for line and clearing-out signals in toll boards 
instead of drops, but as this scheme has advanced but little beyond 
the experimental stage, all of the circuits shown in connection with 
this description have been drop circuits. 

The design of toll board sections has, during recent years, become 
fairly well standardized, a single typical section being shown in 
Fig. 357. The framework is nearly always of wood instead of iron, 
as in the case of multiple boards, and the woodwork as a rule is of 
mahogany, although oak has been used in some cases. The sec- 
tions are usually of the two-position type with much wider key- 
shelves than it is customary to furnish on sections used for local 
work. This allows the toll operators more space for making out 
tickets and does not hamper their work as, due to the comparatively 
small amount of equipment in a toll board, the "reach" is never 
great. In the face of the board and within the reach of each posi- 
tion is also usually provided a number of pigeon-holes in which the 
toll tickets and various other memoranda may be readily filed away. 

Room is usually left on the key-shelf in the middle of each section 
for the mounting of an instrument known as a calculagraph, this 
containing the time-stamping mechanism by which the elapsed time 
is recorded on the toll tickets. As one calculagraph is furnished for 
each section it is used by the two operators at that section, and such 
double use of the machine is not found to practically interfere with 
its proper working. 



CHAPTER XXVII. 

DETAILS OF MULTIPLE SWITCH-BOARD APPARATUS. 

The designing of the various pieces of apparatus which form 
parts of that wonderful complex whole — the modern multiple com- 
mon battery switch-board — requires attention to an infinite number 
of details. In this chapter some of the most important parts of ap- 
paratus employed in practice to-day will be illustrated and dis- 
cussed, but on account of the great variety and almost numberless 
types of such apparatus a complete treatment of the subject will 
be impossible. 

The spring-jack is perhaps the most important of all pieces of 
switch-board apparatus. This is certainly true in point of numbers, 
as in the multiple board the spring-jacks increase almost as the 
square of the number of subscribers, and are often numbered by 
hundreds of thousands. 

It is now almost the universal practice to build up spring-jacks 
in strips, usually of 20, the framework of these strips being formed 
for the most part of hard rubber, sometimes stiffened by strips of 
brass. The multiple jacks are almost always mounted in strips of 
20, a few cases being on record where strips of 25 have been used. 
This, however, is not to be recommended for several reasons. The 
answering jacks may be mounted in strips of 10 or 20, 20 being 
common in boards where the jacks are not made closer together 
than on J-inch centers. Where, however, the spring-jacks are made 
smaller than this, it is better to mount the answering jacks 10 per 
strip rather than 20, thus giving more room between them and 
preventing undue crowding among the answering plugs, which, be- 
sides being inconvenient for the operator in handling, may also 
serve to hide the signals from her view. 

A jack used to a large extent by the Western Electric Company 
is made on f-inch centers, both horizontal and vertical for the mul- 
tiple. The answering jacks are spaced on f-inch centers, horizon- 
tally, the strip itself being f-inch high. 

The construction of this jack is shown clearly in Fig. 358. which 
needs little description. The "tip" and "ring" contacts consist of the 
two springs which register with the tip and ring contact on the 

511 



512 



AMERICAN TELEPHONE PRACTICE. 



plug. The test or sleeve contact of the jack is of German silver, 
provided with a rearwardly-extending shank projecting from the 
rear of the jack strip for the purpose of allowing the soldering of 
the wires. This jack represents the highest development in the 
construction of a three-contact jack for multiple switch-board 
work. There are no make-and-break contacts between the jack 
springs, all contacts being normally insulated from each other and 
adapted to engage only with the plug contacts during use. On ac- 
count of considerations as to mechanical strength, and as to the 
proper insulation between the parts, it has not been found expe- 
dient up to the present time to build three-contact jacks closer to- 





ne 35S.— WESTERN ELECTRIC JACK. 



gether than on f-inch centers, and, in fact, considerable difficulty 
was experienced in securing the design of a jack with three con- 
tacts which would be mechanically and electrically reliable when 
made as small as this. The jacks of Fig. 358 are those forming a 
portion of a strip of 10 answering jacks, and are therefore mounted 
f-inch apart horizontally. The notches in the front of the strip 
are for the insertion of designation plates by means of which the 
answering jacks may be identified. 

This will be recognized as the spring-jack which is used in con- 
nection with nearly all of the Western Electric common battery 
circuits. 

In Fig. 359 is shown a strip of two-point jacks, which is of the 



MULTIPLE SWITCH-BOARD APPARATUS. 



513 



type used in nearly all of the two-wire switch-boards installed by 
the Kellogg Company. 

Detail views showing the construction of this jack are shown 
in Fig. 360, from which it will be seen that the contacts of the jack 
consist merely of a tip spring and a sleeve contact, these two hav- 




FIG. 359.-STRIP OF TWENTY KELLOGG TACKS. 

ing rearwardly-projecting portions for facilitating the soldering of 
the wires. The framework of this jack is composed mainly of two 
hard-rubber strips, the front strip being drilled to receive the tubu- 
lar portion of the sleeve contacts, while the rear strip is slotted on 
its upper side to receive the tip springs, and on its lower side to 




FIG. 360.— DETAIL OF KELLOGG JACKS. 



receive the shanks projecting from the sleeve contacts. Under- 
neath this rear rubber strip is secured a heavy brass strip into which 
the screws binding the tip springs in place pass. A thin rubber 
strip is placed between the slotted rubber back strip and the brass 
strip, this serving to prevent the sleeve contacts from short-circuit- 
ing against the brass strip. Grooved end lugs of brass secured to 
33 



514 



AMERICAN TELEPHONE PRACTICE. 



the upper face of the back strip and the rear face of the front strip 
serve as a means for fastening these spring-jacks into the frame- 
work of the board. 

This jack of the Kellogg Company is made in three sizes, the 
largest of which is -J inch between centers in a horizontal direction 
and 7-16 inch in a vertical direction. This size is used by the Kel- 
logg Company on all switchboards having an ultimate capacity of 
not over 7200 lines. There is another size — § inch between cen- 
ters in each direction — which is used for exchanges having an ulti- 
mate capacity of not over 12,000 lines. The smallest type yet put 
in practice is mounted on 3-10-inch centers in each direction, this 
being embodied in the large multiple boards of the Frontier Tele- 
phone Company of Buffalo, N. Y., and of the Home Telephone 



Mmmmi 



, OOOOQDOOOOpOOOOpOOC 

oooocboooopoooqooooo, 
iooooaDoooopooomoooa 



FIG. 361.— STROMBERG-CARLSON MULTIPLE JACKS. 

Company of Los Angeles, Cal. The ultimate capacities of these 
switch-boards in each case is 18,000 lines. 

The Stromberg-Carlson Company have recently made a some- 
what radical departure from what is generally considered standard 
construction by mounting their multiple jacks in banks of 100 in- 
stead of in strips of 20. A good idea of one of these banks is af- 
forded in Fig. 361, it being seen that the front strip, which is of a 
solid piece of rubber, carries 5 rows of jack sockets, there being 20 
jacks in each row. The rear strip forms a support for the vari- 
ous springs and terminals and is secured to the front strip by means 
of heavy brass end lugs, which also form the means of securing 
the bank into the framework of the board. Even in their 
two-wire multiple boards the Stromberg-Carlson Company use 
three contacts in each jack. The jack has a ring contact and two 
springs, the ring contact and the forward spring being permanently 



MULTIPLE SWITCH-BOARD APPARATUS. 515 

tied together in the jack, thus making one contact of the two. 
Their idea in doing this is to afford what they consider a more se- 
cure contact between the sleeve of the plug than would be afforded 
by a tubular contact alone. 

All of the jacks shown, it will be seen, are arranged in horizontal 
rows of 20, as that number has been adopted as a universal unit for 
the interior wiring of telephone exchanges. Twenty jacks form 
a convenient unit and a bank of ioo is usually formed by piling 
5 strips together in the board. The arrangement for the number- 
ing of the jacks brought about by this method makes easier the 
selection of any number in a bank of ioo than would be the case 
with any other available unit. Mounting the jacks in strips of 25 
has been tried, notably by the Sterling Electric Company, in an 
exchange installed by them in Toledo, Ohio, in which case 4 strips 
are used for each bank of 100. With this arrangement, however, 
the operators cannot as readily select a number in the bank, be- 




ne 362.-NUMBERING OF MULTIPLE JACKS. 

cause, since the strips are not in multiples of 10, a given final figure 
of a number will occupy different positions in the various strips 
forming a bank of 100. In the numbering adopted in practice with 
5 strips, 20 jacks to a strip, the first strip, beginning at the top, is 
numbered from o to 19, the second from 20 to 39, and so on, the last 
being from 80 to 99, thus making the jacks in a bank of 100 appear 
as shown in Fig. 362. In the multiple, however, the first hundreds 
are placed in the lowest row, with higher hundreds above. 

In order to facilitate the work of the operators in selecting a 
number quickly, it is customary to divide off each strip of jacks 
into four equal parts by means of either dots between the 5th and 
6th, ioth and nth, 15th and 16th jacks in each strip, or else by ver- 
tical lines, as is clearly shown in the cut of the Stromberg-Carlson 
jack in Fig. 361. 

A multiple cable, in the case of the 3-wire jack of the Western 
Electric Company, is made from 63-wire cable, this consisting of 
21 twisted pairs and 21 single wires, the twisted pairs, of course. 



516 AMERICAN TELEPHONE PRACTICE. 

carrying the tip and ring contacts forming the talking circuit, and 
the single wire carrying the test or sleeve contacts. In the case 
of the Kellogg Company's 2-wire system the multiple cable is 
formed of 21 pairs (42 wires). In each of these cases a separate 
cable feeds each strip of 20 jacks. 

In the Stromberg-Carlson 2-wire system, where the multiple 
jacks are mounted in banks of 100, the multiple cable is formed 
of 102 pairs of wires, a separate cable thus feeding each bank of 
100 jacks. 

The answering jacks in nearly all systems are the same as the 
multiple jacks in individual construction, but they are frequently 
mounted in strips of 10 instead of 20, the length of a strip being 
the same in each case, but the space between the jacks being twice 
as great in the former case as in the latter. 

A form of lamp jack, used very largely until recently, was shown 




FIG. 363.-NORTH ELECTRIC COMPANY'S LAMP JACK. 

in Fig. 245. This particular strip is one of 20 jacks as formerly 
manufactured by the Kellogg Company, a strip being formed of a 
single piece of hard rubber, milled and drilled as shown. The main 
portion of this strip is slotted from above and beneath for the pur- 
pose of securing in place and affording room for the contact springs 
for the lamp. After a lamp is put in place between the springs a 
small lens of opalescent glass, which is carried in a brass bushing, is 
pushed into place in front of the lamp. 

Recently nearly all of the manufacturing companies have adopted 
a lamp strip constructed with a metal framework, a good example 
of which is shown in Fig. 363, which represents a strip of 10 lamp 
jacks as manufactured by the North Electric Company, of Cleve- 
land, Ohio. This strip, as will be seen, is partly of metal, but is 
provided with a hard rubber face through which the lamps are in- 
serted into the jack, and into which the lamp caps are afterwards 



MULTIPLE SWITCH-BOARD APPARATUS. 



517 



pushed. The lamp terminals and springs are in this case mounted 
'on edge in a rear strip, secured to the front by means of a brass plate 
underneath, which stiffens the whole structure. This cut also shows 
views of the lamp, the lamp extractor and the cap and cap extractor 
in common use. 

The Stromberg-Carlson Company have' recently introduced a 
combined answering jack and lamp strip, as shown in Fig. 364. 
This feature is one of apparent merit, as lamp jacks are always as- 
sociated with answering jacks, and there is therefore no good rea- 
son why they should not be built together. 

As will be seen, the answering jacks are beneath the lamp jacks, 
the two having a rubber face in common. 




FIG. 



.—STROMBERG-CARLSON COMPANY'S LAMP AND 
ANSWERING JACK. 



Both the Kellogg Company and the Stromberg Company, in their 
recent exchanges, have adopted the scheme of discarding the opales- 
cent lens in front of the line lamp, using in its place a thin, flat glass 
disk, behind which and on a piece of paper is printed the number of 
the line. As will be pointed out in a later chapter, changes on the 
intermediate distributing board often destroy all significance as to 
the numbers of the answering jacks. By having the lamp cap carry 
the number, the number of the answering jacks may, with very little 
trouble, be made to conform with that of the line. This method of 
numbering is quite clearly shown in Fig. 364. The Bell companies 
use a separate designation plate for the answering jacks, these plates 



518 



AMERICAN TELEPHONE PRACTICE. 



being made removable in order that changes may be made in the 
numbering. 

Fig. 365 shows the relations between the answering jacks and 
lamps, and between these and the multiple jacks as mounted in a 
Stromberg-Carlson multiple board. In the panels just above the 
plug shelf are seen the pilot lamps, these being provided with large 




ftym&M 




aft 



FIG. 365.— RELATION BETWEEN ANSWERING AND MULTIPLE JACKS. 



lenses in order to give a more brilliant signal. This cut also shows 
well the mounting of the answering and calling plugs, together with 
the ringing and listening keys of each cord circuit. The supervisory 
lamps are mounted just back of the ringing keys in the metal plate 
there shown, there being one of these for each plug. It will be 
noticed that in this cut the dividing lines, or stile strips, between the 



MULTIPLE SWITCH-BOARD APPARATUS. 519 

multiple and answering jacks do not correspond. This is not in 
accordance with the usual and best practice, in which the strips of 
answering jacks and lamps are made to be of the same length as 
those of the multiple jacks, thus allowing stile strips, which divide 
the spaces between the jack strips, to run continuously from the 
keyboard to the top of the multiple jack space. This is, of course, 
desirable from the standpoint of rigidity of construction. 

As there are usually three operators' positions to each section, the 
practice of making the panels in which the answering jacks are 
mounted the same width as those in which the multiple jacks are 
mounted prevents the answering jack panels from being evenly ap- 
portioned among the operators, unless it happens that the number of 
panels in a section is an exact multiple of three. The number of 
panels in the multiple is usually determined by the size of the section, 
as limited by the reach of the operators and by the length of the jack 
strip, as limited by the size of the jacks. It is found that the best 
length for a section is in the neighborhood of 67 inches, 65 inches 
being perhaps the minimum, and 70 inches the maximum, as 
allowed by good practice. Coming within these dimensions for the 
length of the section when jacks are mounted on J-inch centers, 
there are usually six panels to a section; when jacks are mounted on 
f-inch sections there are usually eight panels to a section, and when 
mounted on 3-10-inch centers there are usually ten panels to a sec- 
tion. When, therefore, the jacks are mounted on -J-inch centers 
giving a six-panel section, the stile strips accurately divide the posi- 
tions between the operators; in that case there are two panels to each 
position. This apparently affords a slightly better arrangement in 
regard to the answering jacks and lamps, as the operators' field of 
operation with respect to these jacks and lamps is somewhat better 
defined. This advantage, however, is more apparent than real, for, 
as a matter of fact, the work of the operators in answering calls "laps 
over" to a considerable extent, as sometimes an operator in one 
position will be very busy, while the one adjacent to her will have 
little to do, in which case the latter operator may reach across with 
her plugs and cords and help the busy operator to handle the work. 
It is therefore thought that the practice of dividing the answering 
jack panels in a different manner from the multiple jacks, in order to 
make them conform more closely to the operator's positions, is not 
desirable, as it has the obvious structural disadvantage of breaking 
up the stile strips, and thus making the face of the board less rigid. 

In Fig. 366 is shown the iron framework of a multiple switch- 



520 



AMERICAN TELEPHONE PRACTICE. 



board section, this being a view of the same section, stripped of its 
apparatus, which is shown in detail in Fig. 365. As is seen, the 
frame is built of structural iron. Much of the woodwork shown 
in place in this figure is used as a medium on which to mount some 
of the apparatus and wiring. This represents about the condition of 
the multiple switchboard section when shipped from the factory, 



■■■■■■il 




FIG. 366.— FRAME OF MULTIPLE SECTION. 

most of the apparatus and the enclosing woodwork being, as a 
rule, fitted to the section before leaving the factory, but afterward 
taken off and shipped separately. 

In Fig. 367 is shown in detail the mounting of the supervisory 
lamps in the Stromberg-Carlson switchboard, this being in con- 
formity with good modern practice. The lamp-jack itself is screwed 
to the under side of the key shelf, the lamp projecting upwardly from 



MULTIPLE SWITCH-BOARD APPARATUS. 



521 



a hole bored in the shelf, the cap carrying an opalescent lens screwed 
into place in the woodwork above the lamp. 

> In the case of supervisory lamps it is found necessary to afford 
some means of orotecting the lenses from breakage due to possible 




FIG. 367.-SUPERV1SORY LAMP MOUNTING. 

impact from the plugs, and for that reason a perforated metal shield 
is arranged in connection with the brass portion of this cap, as shown 
in this cut. 

It has been said that the same type and size of lamp is used for 
pilot lamps as for line and supervisory lamps, but that in order to 




FIG. 



.-PILOT LAMP MOUNTING. 



make a more luminous signal a larger and better lens is provided. 
The construction of the pilot lamp mounting, as used by the Strom- 
berg-Carlson Company, is shown in Figs. 368 and 360. which shows 
the mounting assembled and in its various parts. The lamp jack 
itself in this case mounts on the rear face of the wood panel at the 



522 



AMERICAN TELEPHONE PRACTICE. 



lower portion of the jack space, while the large brass bushing which 
serves to carry the lens screws into the face of the board from the 
front. 

The construction of switchboard cords and plugs has been taken 
up to some extent in previous chapters, and the practice in this does 
not differ materially as applied to multiple boards from that as ap- 




FIG. 



^.-DETAILS OF PILOT LAMP MOUNTING. 



plied to small boards. It, of course, "goes without saying," how- 
ever, that a two-conductor cord will give less trouble than a three- 
conductor cord, and a two-conductor plug less trouble than a three- 
conductor plug. Moreover, it is possible to make a stronger two- 
conductor plug than a three-conductor when the sizes are kept the 
same. So far as the consideration of the cords and plugs per se is 
concerned the advantages are all in favor of the use of the two con- 





FIG..370.-PLUG END OF CORD. 

ductors, and this is one of the important advantages of the two-wire 
multiple board systems. 

The method of preparing the terminals of the cord for attachment 
to the plug is shown in Fig. 370. In this the upper cut shows the 
cord itself before the small hook-shaped tip in the lower cut is ap- 
plied. The tip strand of the cord is, it is seen, wound with wire, 
leaving a small portion of the tinsel conductor projecting through 



MULTIPLE SWITCH-BOARD APPARATUS. 523 

it, and to this wire wrapping is clamped a small hook connector, 
shown in the lower portion of the cut. After this a drop of solder is 
applied at a high enough temperature to make it run, but not high 
enough to burn the tinsel. This secures permanently the electrical 
connection. The end of the sleeve conductor of the cord is lapped 
back outside of the outer braiding, and this when screwed into the 
shank of the plug makes a firm connection between the sleeve con- 
tact of the plug and the sleeve strand of the cord. The connection 
between the tip conductor of the cord and the tip contact of the plug 
is made by means of a screw engaging the hook-shaped terminal of 
the tip strand, this being more clearly shown in Fig. 166. 

In order to prevent the short-circuiting of the tip and sleeve con- 
tacts of the jack when the plug is inserted, a plug constructed as 




m 



FIG. 371.-TWO-CONDUCTOR PLUG FOR MULTIPLE SWITCH-BOARD. 

shown in Fig. 371 is used by the Kellogg Company. In this be- 
tween the tip and sleeve conductors of the plug there is a small 
metal ring, insulated from both tip and sleeve conductors. Elec- 
trically this ring exercises no function as a conductor, and might be 
of insulating material. It is found, however, that insulating material 
in this place will not stand the wear of the constant insertion and 
withdrawal of the plug, therefore an insulated metal ring is used. 

The end of the cord opposite the plug end is usually provided 
with some metallic clip or terminal, such as shown in Fig. 372, this 
particular cut representing the Kellogg method. The method of 
applying this terminal is similar to that described in applying the 
tip strand terminal to the plug end, as shown in Fig. 370, but in this 
case the metal terminal is fork-shaped and is squeezed tightly 
around the wrapping of wire on the outside braid of the cord. 



524 



AMERICAN TELEPHONE PRACTICE. 



The tinsel projects through this and is soldered to the terminal, as is 
clearly shown. The switchboard cord, where this is of the two- 
conductor type, forks at the end opposite the plug end, and at this 
fork the braid is usually extended in the form of a loop, which may 




FIG. 372.— RACK END OF CORD. 



be hooked on to the cord-connecting rack to form a support for 
the cord, which will relieve the conductors from strain due to the 
cord weights. 

The method of connecting the cord to the connecting rack is 
shown in Fig. 373, which shows two stationary terminals secured to 
the rack, to which are secured by screws the fork-shaped cord ter- 




FIG. 373.— CONNECTION OF CORD ON CORD-RACK. 

minals shown in Fig. 372. The hook and loop supporting the 
weight of the cord are clearly shown underneath this rack. The 
wiring leading to the other portions of the cord circuit terminates in 
a soldered connection at the upper portion of the stationary clips, 



MULTIPLE SWITCH-BOARD APPARATUS. 525 

this arrangement affording a ready means for changing the cord in 
case of defect, without disturbing any of the other wiring. 

This brings us logically to the question of cord weights, a typical 
form of which is shown in Fig. 374. This consists of a weight com- 
posed of a hard alloy of lead cast about the lower portion of the 
brass punching, in which is secured a pulley through which the cord 
runs. The weight of these cord weights is, as a rule, about 8 
ounces, that amount of weight being required to restore the cords 
properly to their places when the plugs are withdrawn from the 
jacks. The weights should not be much heavier than this, on 




FIG. 374.-CORD WEIGHT. 

account of the obvious disadvantage of subjecting the cord to too 
great strain and to consequent wear. 

It is thought that there has been no more difficult type of tele- 
phone apparatus to properly design than that comprising the va- 
rious ringing and listening keys, by which the operator is enabled 
to bring her telephone and generator into connection with the 
various cord circuits. Something has been said of these keys in an 
earlier chapter, but several of the types in most common use will be 
shown here, this forming an important part of multiple switchboard 
work. 

In Fig. 375 is shown one of the best and most substantially con- 
structed types of keys yet produced, this being that of the Western 
Electric Company. Nearly all of the keys of this company are of 



526 



AMERICAN TELEPHONE PRACTICE. 



this general type, the particular one shown being adapted to two- 
party line ringing and listening. 

The two operating levers, L and L', are pivoted in a heavy brass 
casting, A, upon which the several sets of contact springs are 
mounted. The button or wedge of insulating material (rawhide) 
carried on the lower end of the lever, L, plays between the listening 
group of springs, shown at the left of the figures, and a set of ringing 
springs, which is the middle group on the key. When, therefore, 
the key lever, L, is thrown to the right, as seen in the cut, the oper- 
ator's telephone set will be connected with the cord circuit. When 




FIG. 375.— WESTERN ELECTRIC RINGING AND LISTENING KEY. 

this lever is thrown in the opposite direction the calling plug is dis- 
connected from the rest of the cord circuit and is placed in proper 
relation with the calling generator terminals to effect the ringing of 
the bell of one of the parties on the line with which the calling plug 
is connected. The wedge of the lever, L, plays between one set of 
springs only, this lever when thrown to the right being adapted to 
serve as a ringing key for the other party on the line. 

The operation of the springs themselves will be obvious from 
what has already been written of the cord circuits of the W T estern 
Electric Company. The small hook-shaped spring on the inside of 
the group of listening springs is that used for completing the cir- 



MULTIPLE SWITCH-BOARD APPARATUS. 527 

cuit of the restoring coil of the clearing-out drop when the listening 
key is thrown. 

The upper cut in Fig. 375 shows a top view of the key with its 
upper hard rubber finishing strip removed. This discloses a top 
view of a lever, a, carrying a white and red target, b. This target is 
displayed to the view of the operator through a hole in the rubber 
finishing strip. The lever, a, is so arranged as to be moved by the 
slotted link rods, c and d, pivoted respectively to the key levers, L 
and JJ . By virtue of the pin and slot connection between these 
links and the lever, a, the target will be moved into the position 
shown (and will therefore appear white) when the lever L is thrown 
into its ringing position. It will not be moved by subsequent 
operation of this lever, but if lever L' is thrown the target will be 





FIG. 376.— KELLOGG RINGING AND LISTENING KEY. 

moved by means of link, d, so as to have its red portion visible to the 
operator. This target, therefore, always shows to the operator 
which one of the ringing keys was last operated, so that if required 
to ring a party a second time she will not press the wrong key. 

It is obvious that this key may be modified to meet almost any 
requirements of service, as for instance, by leaving off the right-hand 
cam with its group of springs, and also the indicating device, the key 
becomes merely a combined ringing and listening key for ordinary 
single-party work. This key is secured in the key shelf in an iron 
framework, the various keys being mounted close together side by 
side and secured in this framework by machine screws. Eighteen 
of such keys may be placed side by side in an ordinary key shelf 



528 



AMERICAN TELEPHONE PRACTICE. 



without unduly crowding, and at the same time leaving enough 
room for order-wire keys, if such are required. 

The type of key used by the Kellogg Switchboard and Supply 
Company in nearly all of their work is shown in Fig. 376. 

In Fig. 377 is clearly shown how two or more of these keys may 
be associated on a single mounting strip for the purpose of securing 
any desired combination of keys. The means by which this key is 
made into a two-way ringing key for two-party line service deserves 
attention. No separate indicator is used, but a knuckle joint is pro- 
vided in the handle of the key at the point where it joins the cam. 
This joint, by means of a spring-pressed contact, moves with con- 
siderable friction, but always moves when the operator presses the 




FIG. 377.-TWO KELLOGG KEYS ON SINGLE PLATE. 



key into the ringing position in one direction or the other. The 
handle of the key is always left leaning in the direction in which it 
was last pressed, thus serving as an indicator to the touch, as well as 
to the vision, of the operator. 

The four-party line ringing key of the Western Electric Company 
is shown in Fig. 378. The operation of this key in its relation to 
four-party line signaling will be apparent from the circuits already 
given in the chapter on Party Lines. The fifth ringing key at the 
left in the figure is for ringing on individual lines where no selective 
signaling is required. The cam-key at the right is the listening 
key. The indicating feature for the ringing keys in this is interest- 
ing, consisting as it does of four slidable plates adapted to be 
moved in one direction or the other by the operation of any one of 



MULTIPLE SWITCH-BOARD APPARATUS. 



529 



the plungers. As these plates all bear against each other except for 
the slight space which may be left between any pair of them, and 
since there is only room enough in their slide-way for one such space 
between them as is produced by the movement of the plunger when 




FIG. 378.— WESTERN ELECTRIC FOUR-PARTY-LINE KEY. 

operated, it follows that an opening will always be left between the 
pair of plates corresponding to the plunger last depressed. The 
surface below the plates is painted a bright red, and this showing 




FIG. 379.-KELLOGG FOUR-PARTY-LINE KEY. 

through the opening between the plates serves to indicate to the 
operator what key was last depressed. 

The Kellogg four-party line ringing key is shown in Fig. 379. In 
general form, circuits and use this key is similar to the Western 



530 AMERICAN TELEPHONE PRACTICE. 

Electric Company's key just described, but it has an additional ad 
vantage in recording the last party rung. In the figure, a metal lock- 
ing plate will be observed, pivoted with screws to the vertical pillars 
uniting the top and bottom portions of the key, extending along be- 
side the ringing plungers, and with a small tip projecting through 
the upper finishing plate of the key just between the listening cam 
and the first ringing plunger. This locking plate engages a pro- 
jection on each ringing plunger which prevents a depressed plunger 
from returning to its normal position of rest, and holds it slightly 
below the level of the other plungers of the key, but the pressing of 
any plunger so engages and moves the metal locking plate as to re- 
lease any plunger previously held, thus permitting all plungers to re- 




FIG. 380.— STRIP OF ORDER-WIRE KEYS 

turn to their full height, except the one last used. The operator thus 
may learn by touch, as well as by sight, which station on a line was 
rung last. By pressing the locking plate tip projecting near the 
listening cam, all plungers may be released. In this key, as in any 
key having separate plungers for the separate stations, the push- 
buttons may be made of different colors and may be identified by 
their color as well as by their location on the keyboard. 

Order-wire keys are usually operated in the same manner as push- 
buttons, the plunger when depressed serving to press the two con- 
tact springs with which the terminals of the order-wire line are 
connected into contact with the two springs forming the terminals 
of the operator's telephone set. Such keys are usually mounted in 



MULTIPLE SWITCH-BOARD APPARATUS. 



531 



strips of ten. One of these strips, as manufactured by the Kellogg 
Company, is shown in Fig. 380. 

In this the framework is formed of a solid piece of hard rubber, 
polished on its upper face. The springs are of German silver, 
platinum pointed. Such a strip of keys is usually mounted cross- 
wise on the left-hand portion of the key shelf of the multiple board, 
the face of the rubber block coming flush with the upper face of the 
key shelf. If more than ten order-wire keys are needed, then more 
than one strip is provided, several strips being mounted side by 
side. The approximate dimensions of such a strip as this is 5 inches 
long by -J inch wide. 

Sometimes it is desirable to mount order-wire keys individually, 




FIG. 381.— INDIVIDUALLY MOUNTED ORDER-WIRE KEY. 



and in this case the key shown in Fig. 381 is used, this extending up 
through a circular hole on the key shelf and secured thereto by 
screws passing through the side lugs on the key into the shelf. The 
working parts of this key are mounted in a metal frame supporting 
the hard rubber block on which the several springs are mounted. 

The push buttons or heads of the order-wire keys are usually 
provided with removable caps containing a mica or thin glass top, 
under which may be placed the number or other designation of the 
key or circuit. 

On the front rail supporting the key shelf at the left-hand portion 
of each position of modern multiple boards are mounted one or two 
operators' cut-in jacks. These jacks afford means for instantly 



532 AMERICAN TELEPHONE PRACTICE. 

connecting or disconnecting the operator's head telephone, the free 
end of the cord of which terminates in a plug adapted to enter the 
jack. Besides the tip and sleeve contact for completing the talking 
circuit, these jacks are usually provided with an extra pair of springs, 
open when the plug is not inserted, but closed by the insertion of the 
plug, these springs serving to open or close the circuit which sup- 
plies current to the operator's telephone transmitter. In this way a 
waste of current through these transmitters at idle positions is pre- 
vented. Such a jack and plug are shown in Fig. 382, this being a 
type now manufactured by the Stromberg-Carlson Company. This 
type was originated by the Kellogg Company, and has the advan- 
tage of taking up very little room under the key shelf, the U-shaped 
springs extending a very much less distance back under the key 
shelf than if the springs were straight. When two such jacks are 




FIG. 382.-CUT-IN JACK AND PLUG. 

mounted side by side they are wired together in multiple, the object 
being to provide a ready means for the instruction of new operators, 
and also for the repair men or inspectors to listen in without inter- 
fering with the work of the regular operators. 

The support of the operators' transmitters in telephone switch- 
boards has been the subject of much thought, and opinion and 
practice are now divided between two general methods. One of 
these, used to but a comparatively slight extent in this country, 
although finding considerable favor in Europe, is to support the 
transmitter directly on a plate carried on the operator's breast, this 
plate being suspended by a band passing around her neck. A long 
mouth-piece is provided extending to a point within a few inches of 
the operator's lips. Such a type has the advantage of not obstruct- 
ing the view of the operator of the face of the board and of auto- 
matically following the operator in her motions. It, however, has 



MULTIPLE SWITCH-BOARD APPARATUS. 



533 



the drawback of being in the nature of additional "harness" for the 
operator to wear, of not securing as good transmission, because of 
the long curved mouth-piece, and of requiring that the operator's 
cut-in plug and jack carry four contacts instead of two, two for the 
transmitter and two for the receiver. It is also found rather difficult 
to prevent these transmitters from becoming foul, due to the con- 
stant breathing of the operators into the mouth-pieces. 

The other method of supporting transmitters which, in modified 
forms, is the most largely used in this country, is shown in Fig. 
194. The board upon which the stand is mounted is the roof board 




FIG. 



L— OPERATOR'S TRANSMITTER MOUNTING FOR MULTIPLE 
BOARDS. 



of the switch-board section. As will be seen, the horizontal arm 
from which the transmitter is suspended is, by means of a thumb- 
screw, carried on the top of a vertical post, adjustable in or out, that 
is, toward or from the face of the jack space. The transmitter itself 
is adjustable up and down at the will of the operator, the transmitter 
cords passing over pulleys as shown. Counter-weights hanging on 
the cords within the switch-board section serve to balance the 
transmitter so that it will remain in any position. 

Fig. 384 shows a transmitter suspension best adapted to large 
multiple boards. In this the upright post is abandoned, the hori- 



534 AMERICAN TELEPHONE PRACTICE. 

zontal arm passing through a brass bushing in the frieze-board 
above the multiple jacks. 

The relays in telephone work assume an almost endless variety of 
forms. There has been a gradual evolution in these from the type 
used in telegraphy, commonly known as the Morse relay, one of 
which is shown in Fig. 385. For a quick-acting relay and a very 
sensitive one this type is still used in telephony where there is no 
particular need to economize room. The evolution spoken of, how- 
ever, has been made necessary primarily on account of the necessity 
for the economy of room, and also in order to produce a somewhat 
cheaper design than that of the old Morse relay. Of course, other 
factors enter into the problem also. In particular cases, some relays 



ML*J Oh 




FIG. 385.— MORSE RELAY. 

must be of such design as to enable them to be mounted closely to- 
gether without producing cross-talk between them; some relays 
must be polarized so as to operate on current in one direction only; 
others must be differentially wound; some must be designed for 
maximum impedance ; others for no impedance under certain condi- 
tions; some relays, as is the case with the old Morse, are required to 
make one contact only; others are sometimes required to make three 
contacts and break three; sometimes the order in which the contacts 
are made and broken is an important factor in the design. From all 
of this it will be seen that the design of a telephone relay must be 
made with a view to the particular use to which that relay is to be 



MULTIPLE SWITCH-BOARD APPARATUS. 



535 



put, and that a relay adapted to one purpose in a telephone system 
will perhaps be totally unfit for use in some other portion. 

Taking up first the matter of supervisory relays, one of the first 
of these to be developed is of the type shown in Fig. 386, this type 
having been widely used in Bell exchanges. 

The coil, b, of this relay was in the earlier types enclosed in a heavy 
iron shell, the armature, d, serving when attracted to completely 
close this shell. The armature itself is in the form of a truncated 
cone, and has no support save that it rests within the cup-shaped cap 
which fits over the front end of the shell and protects the working 







FIG. 



-OLD BELL SUPERVISORY RELAY. 



parts of the relay from dust. When the coil is not energized the 
armature falls back by gravity against the adjustable stop carried in 
the cap, while, when attracted, it merely rocks on its knife-edge sup- 
port and completely closes the shell. This is a single-contact relay, 
the stationary contact being carried on the front end of the core, this 
end of the core being hollowed out, as at a, and provided with an 
insulating bushing carrying a brass clip, upon which is mounted the 
platinum contact. The armature itself carries at its center point a 
screw which is platinum pointed and adapted to make contact with 
the stationary contact point when the armature is attracted. Such 



536 AMERICAN TELEPHONE PRACTICE. 

relays were mounted on suitable iron strips on about if-inch centers, 
such a strip of ten being shown in Fig. 387. 

In later types of this relay, the heavy iron shell was abandoned, 
and the structure of shell specifically shown in Fig. 386 adopted. 
The primary object of the iron shell is to prevent cross talk by con- 
fining the lines of force set up in the coil of the relay, within the 
structure of the relay. It was later found by McBerty that a copper 
shell was effective for this purpose, and therefore a thin iron tube, c, 
Fig. 386, merely bent up from sheet iron, was used as a return cir- 
cuit for the magnetic lines of force set up in the core, while cross 
talk was prevented by the outer shell, f, of thin copper. The applica- 
tion of the McBerty copper shell to coils of various kinds used in 
telephone work, was an important step in the art. It allows a much 
greater latitude of design of coils for various purposes. 

This type of relay, while effective and efficient, is not now largely 




FIG. 387.-STRIP OF SUPERVISORY RELAYS. 

used except in comparatively few installations of the Bell companies. 
It had one serious fault in that the knife-edge contact between the 
armature and the inside of the case formed a part of the local circuit 
to be closed by the relay, which contact was not reliable. It was 
made so, however, by bridging the contact with a little spiral of 
wire soldered at one end to the shell and at the other to the arma- 
ture, but this was a nuisance, and was bad mechanically. More- 
over, this type of relay was adaptable only to the operation of a 
single "make" contact, being useless when it became necessary to 
provide for both making and breaking one or several contacts when 
energized. 

The type of relay now used for line and supervisory work by the 
Western Electric Company in the switch-board they manufacture 
for the various Bell companies is shown in Fig. 388. 

This is a modification of the tilting armature type shown in Fig. 



MULTIPLE SWITCH-BOARD APPARATUS. 



537 



386, and needs little explanation. For supervisory purposes where 
it is important to prevent cross talk, the outer shell, g, of this relay 
is made of copper about 3-32 inch thick. This type of relay is also 
used as a line relay for lighting the line lamp, but in this case the 
copper shell is omitted, and all of the relays on the strip are enclosed 
in a common dust-proof case. 

The type of relay now largely used by the Bell companies for 
their cut-off relay, or, in general, where several contacts have to be 
made and broken, is shown in plan, elevation and section in Fig. 

389. 

In this, m, is the mounting strip to which is secured the core, a, 
carrying at its rear end an angular pole-piece, d, and at its front end 




d . 


m 










v^ 





FIG. 388.— WESTERN ELECTRIC LINE RELAY. 



a similar piece, d' ; secured to the under side of the pole-piece, d, by 
a spring, e', is an armature, e, projecting forwardly under the hori- 
zontal portion of the angular pole-piece, d' . This armature is there- 
fore raised by the magnetic pull of the two pole-pieces when the 
coil is energized. The armature thus serves to complete the mag- 
netic circuit between the two pole-pieces. The downward move- 
ment of the armature is limited by the adjustment screw, f, against 
which it rests when not attracted. The range of movement of the 
armature may be adjusted by means of this screw, as is readily soon. 
Carried on the upper face of the pole-piece, d, are two pairs of con- 
tact springs, i and g, i' and g\ these springs carrying platinum con- 
tact points at their forward ends, and are so arranged that the upper 



538 



AMERICAN TELEPHONE PRACTICE. 



spring will normally make contact with the lower one. The lower 
spring, i, in each case rests on a rubber bushing on the front pole- 
piece, d', thus affording a permanent support for these springs, 
which have a downward tension against these bushings. The 
movable springs, g, also have a downward tension, which serves 
to keep their platinum contacts in engagement with the lower 
springs. Two rubber plugs, k and k\ are carried by the armature, 
e, and one of these extends through each of the lower contact 
springs, i or i', the holes in these springs through which these plugs 
extend being considerably larger than the plugs themselves. When 





M 



W 



f 1 .'.'. ' I i B^B mi f' ■ I n j? ' 




i 



FIG. 389.— WESTERN ELECTRIC CUT-OFF RELAY. 



the armature is attracted the plugs, k, serve to raise the springs, g, 
thus breaking their contact with the springs, i. 

It is easy to see that the springs, g, in their upward movement 
might also be made to make contact with other springs above them 
when the armature is attracted. In order to shield the entire re- 
lay from dust a sheet-iron shell, n, slips over the entire structure and 
is held in place by a nut, g, permanently secured to the shell which 
engages a screw, o, carried on the frame of the relay. This is the 
type of relay now used by the Bell companies as a cut-off for their 
line circuits in common battery work. 



MULTIPLE SWITCH-BOARD APPARATUS. 539 

A type of relay used by the Kellogg Company for both its line 
and cut-off relays is shown in Fig. 390. 

In this it will be seen that the springs are mounted in a group or 
groups on an iron plate carried on the top of the coil, this plate being 
bent downwardly at its rear end and engaging the rear of the core. 
Resting on the knife-edge on the front of this plate which extends to 
the front of the relay coil is an angular armature which, when its 
lower end is attracted, raises the long spring out of contact with the 
lower spring and into the contact of the upper one. The lower spring 




FIG. 390.-KELLOGG LINE RELAY. 

rests on the shoulder of a hard rubber post, upon the top of which is 
also supported the upper springs, both of these springs having a 
normal tension downward. The hard rubber post passes through a 
large hole in the long spring, so as to allow its free movement up 
and down. By mounting three groups of springs on these relays 
as many as three make and three break contacts may be secured. 

In the Kellogg line relay but a single "make" contact is required. 
there being one group of springs mounted on the center of the 
upper plate, these being adapted to make contact when the arma- 
ture is attracted. 



540 



AMERICAN TELEPHONE PRACTICE. 



In the Kellogg cut-off relay, as used in the line circuit of the Kel- 
logg multiple switch-boards, there are two groups of three springs, 
each having one make and one break contact. This relay, in what- 
ever type made, is enclosed in a drawn iron shell about 1-16 of an 
inch thick, this shell being screw-threaded on the circular washer 
forming the base upon which the relay parts are mounted. 

A group of twenty line and cut-off relays, ten of each mounted on 
a single strip of cold drawn steel, is shown in Fig. 391. This illus- 
trates a feature of modern practice as adopted by nearly all com- 
panies, that of mounting the line and cut-off relays in such proximity 
as to enable much of the wiring to be done with short, stiff, bare 
jumper wires extending between the terminals of the relays them- 
selves on the strip. The other side of this strip has the relay ter- 




FIG. 391.— STRIP OF TWENTY LINE AND CUT-OFF RELAYS 



minals so connected that all wiring is open and ready for inspection, 
the use of cable being very largely eliminated. 

The Kellogg Company is also manufacturing a somewhat cheaper 
and more compact relay, which it terms its "minor" relay, which 
embraces the same mode of operation as its major type of relay just 
described, but embodies some very unique features in point of con- 
struction. 

These relays are of the tubular type, but the method of construc- 
tion differs radically from that of the ordinary tubular relay, for a 
single iron casting is drilled out to take the place of ten tubular 
shells. The various contact springs are arranged directly on top 
of this iron casting, being secured thereto by screws passing 
through rubber bushings and blocks for the purpose of insulating 



MULTIPLE SWITCH-BOARD APPARATUS. 541 




w* 




& » 






FIG. 393.— STRIP OF TEN KELLOGG MINOR RELAYS— FRONT VIEW. 




FIG. 394.-STRIP OF TEN KELLOGG MINOR RELAYS-REAR VIEW 



542 AMERICAN TELEPHONE PRACTICE. 

them from each other and from the iron block. The coils are 
placed directly in the holes bored in the face of the block, their 
terminals extending through small bushed holes in the rear. The 
angular armature hangs over the front upper corner of the block 
in such position that when attracted by the core of the coil within, 
its rear leg will serve to raise the contact springs. In the block 
shown the relays are alternately line and cut-off. Fig. 393 shows 
the front and Fig. 394 the rear of such block of relays, the latter 
figure making clearer the arrangement of the various terminals. 
These relays, as will be seen, are not in themselves dust proof, and 
when therefore this type is used they are either mounted on a rack 
contained in a glass case or each strip of relays is provided with a 
sheet-iron dust-proof box, which slips over the entire strip. 




FIG. 395.— STROMBERG-CARLSOX RELAY. 

The Stromberg-Carlson Company are using a relay almost iden- 
tical in design with the major type of Kellogg relay, the only dif- 
ference being that they provide an adjustable iron screw on the 
lower end of the armature by means of which the air gap may be 
varied. One of their relays without its shell is shown in Fig. 395. 

In the design of pilot relays, which are used in the common por- 
tion of a circuit extending from the battery to a number of different 
circuits, it becomes necessary to design a relay which will be of low 
enough resistance to allow sufficient current to be supplied to a 
number of such circuits in multiple. When such circuits contain 
lamps the pilot relay must be of great enough carrying capacity and 
low enough resistance to prevent causing enough drop through its 
coil to appreciably diminish the voltage of the lamps. At the same 



MULTIPLE SWITCH-BOARD APPARATUS. 



543 



time it must be sensitive enough to be operated by current through 
one lamp only. It must have as another requirement a capacity 
to carry the current required by the greatest number of lamps that 
will have to be lighted through it without undue heat. As com- 
paratively few of these relays are required, the heating effect may 
be taken care of by making the relay of large dimensions, so as to 
have a large radiating surface. This also affords greater winding 
space, so that the required number of turns may be put on the coil 
with comparatively large wire. Such relays usually have a resist- 
ance of from one to three ohms. 

In Fig. 396 is shown the pilot relay of the Kellogg Company, 




FIG. 396.-KELLOGG PILOT RELAY. 



this being of the same general type as its major relay, except that 
the contacts are carried on the front leg of the armature and on the 
front head of the core. This relay has a gravity-actuated armature. 
It is about two inches in diameter and 3J inches long. When 
wound to .022 ohm resistance it will operate with a current of \ 
ampere, and will stand a maximum current of 45 amperes. When 
wound to a resistance of 1.5 ohms it will operate on about 1-20 of an 
ampere, which is amply sensitive for use as a pilot relay, since a 
single lamp requires a current of at least 1-10 ampere to operate it. 



CHAPTER XXVIII. 

POWER PLANTS FOR COMMON BATTERY SYSTEMS. 

The power plant required in an old magneto exchange was an 
insignificant portion of the equipment, and consisted, as a rule, of 
nothing more than one or a few ringing generators driven by some 
constant source of power, and adapted to produce alternating cur- 
rents of the desired voltage and frequency to properly actuate the 
bells at the subscribers' stations. Frequently these machines were 
merely magneto generators provided with exceptionally strong 
fields, and adapted to be driven by the source of power available 
rather than by hand. 

In the old magneto exchanges it was customary in most cases to 
equip each operator's position with a hand generator, to be used in 
case of a break-down of the power ringing machines, so that, aside 
from causing some inconvenience to the operators, the temporary 
disability of the power machines was of little importance. In mod- 
ern exchanges, however, where the switch-boards are of large size, 
and, in fact, in common battery multiple switch-board work in gen- 
eral, the hand generators are not provided, so that entire reliance 
must be placed on the power machines. This, and the fact that the 
operation of the system is also dependent on the proper supply of 
direct current for signaling and talking, greatly increases the neces- 
sity for absolute reliability in the power plant, without which the 
continuous operation of the exchange cannot be effected. 

The most convenient source of primary power for a telephone 
exchange is the mains of some municipal or private power plant, 
when these are available for service at the telephone central office. 
It is not, however, well to place sole reliance for power on a single 
source of such current supply, for a fire in some portion of the city 
or some other catastrophe may result in shutting off the power 
from the mains to the telephone office for a considerable period, and 
as a result render the giving of telephone service to the community 
an impossibility. It is therefore advisable, when possible, to secure 
two entirely independent sources of power, as, for instance, from 
two different power companies, where such are available, or from 
one power company and from an auxiliary power plant operated by 

544 



POWER PLANTS FOR COMMON BATTERY SYSTEMS. 545 

a gas or steam engine, owned or controlled by the telephone com- 
pany. Many telephone companies, therefore, arrange for two such 
sources of power, one of them being usually power mains from the 
outside, and the other a plant operated by a gas or steam engine in 
the telephone plant. In such cases the question as to whether the 
gas or steam engine should be used as the regular source of power, 
and the street mains as a reserve, or vice versa, depends entirely 
on the question of relative cost. This is usually dependent, in large 
measure, on the size of the telephone plant and on the terms the 
telephone company is able to secure from the company supplying 
the power. 

Where electric power is available motors are used usually directly 
connected to the charging generators for delivering direct current to 
the storage batteries and to the smaller alternating current dynamos 
or ringing machines adapted to deliver the proper current for 
ringing the subscribers' bells. In case a gas engine or other prime 
mover is employed, this is generally belted directly to the charging 
machines, which may be of the same type as before, and, in fact, 
the same charging dynamo may be alternately belted to the gas 
engine or driven by an electric motor, according to whether outside 
or local power is being used. 

On account of the necessity for absolute reliability good practice 
dictates that the charging machines, and also the ringing machines, 
should be in duplicate. When two sources of primary power are 
used, each charging machine should be adapted to being driven 
from either source of power. It is customary, in common battery 
offices, to employ two ringing machines, and to drive one of them 
from the street mains or other source of primary power, and to 
drive the other one by a motor deriving current from the discharge 
leads of the storage battery. In this way, in case of a complete 
break-down of all sources of primary power, the telephone plant 
may still be continued in operation as long as the storage battery 
retains a sufficient amount of charge. In a well-designed tele- 
phone plant the storage battery should be of sufficient capacity to 
run the entire exchange for a period of twenty-four hours without 
receiving any additions to its charge. When all of these precau- 
tions have been taken, it is seen that the chance of a complete shut- 
down of the telephone plant, due to any failure on the part of the 
power plant, is extremely remote, and it is a fact that there arc very 
few cases on record of a common battery telephone plant of any 
size ever having been forced to suspend service from this cause. 

35 



546 AMERICAN TELEPHOXE PRACTICE. 

As a rule, nothing short of a serious fire in the switch-board can 
bring about a cessation of service in a telephone office. 

It may be said in general, therefore, that most telephone power 
plants for common battery work comprise at least two direct cur- 
rent machines of suitable voltage and capacity for properly charging 
the storage batteries, and two alternating or pulsating current ring- 
ing machines for operating the subscribers' bells, and that in the 
case of the ringing machines one of them is driven from the same 
source or sources of power which drives the charging machines, 
while the other should be driven by a motor taking current from 
the discharge leads from the storage battery. 

The question as to the source or sources of primary power to be 
used in any exchange is one to be decided only after a careful con- 
sideration of the local conditions in the city in question. Practice 
must necessarily differ widely in this respect, but it may be consid- 
ered as fairly well standardized with respect to the apparatus and 
circuits employed in generating, measuring and controlling the 
charging and ringing currents, as well as the current in the dis- 
charge leads of the storage battery. 

For the purpose of discussing such circuits and apparatus a spe- 
cific case will be taken in the standard power plant and equipment 
furnished by the Western Electric Company for most of its modern 
Bell exchanges. The arrangement of this is shown diagrammatic- 
ally in Fig. 397. 

The power leads from whatever source are shown entering the 
switch-board apparatus in the left-hand side of this figure. In this 
case the available power is direct current at 500 volts, and the mains 
pass first through fuses of sufficient carrying capacity, thence 
through the main switch adapted to cut off the entire power back 
of all apparatus, and thence through arresters for the protection of 
the power apparatus. These consist of an air-gap and a choke- 
coil of low resistance, but high impedance, in each side of the power 
circuit. Each lead then passes through another fuse, and thence 
to the power switch-board proper. An ammeter and a wattmeter 
are here shown for measuring the total current, and the total energy 
delivered to the power board. It is not the practice of the inde- 
pendent companies, as a rule, to furnish the wattmeter on the 
power board, for the reason that this is usually supplied by the com- 
pany furnishing the current, and is mounted by them in some por- 
tion of the power circuit outside of the power board. 

At the point A the main circuit divides into three separate leads, 



548 AMERICAN TELEPHOXE PRACTICE. 

two of which extend through separate pairs of fuses to the single- 
throw, double-pole knife switches, B and C, controlling respectively 
the motors of charging machines, Xos. I and 2, the generators of 
which supply direct current for charging the storage battery for 
the supply of all direct current to the exchange. The third branch 
of the main power lead extends to the knife switch, D, which con- 
trols the motor side of ringing machine Xo. 1, used for supplying 
alternating and pulsating current to the subscribers' lines for ring- 
ing, and also for other minor purposes, which will be pointed out 
later. 

Bridged across the main power leads are the lamps for lighting 
the power switch-board, these being arranged in any suitable man- 
ner, in accordance with the voltage across the power mains. In 
this particular case, where 500 volts direct current is used, five 100- 
volt lamps are placed across the circuit in series, and these are con- 
trolled by a double-pole cut-out of the snap-switch type. 

It is seen that the motors of the charging machines and of ringing 
machine Xo. 1 are provided with ordinary starting boxes, which, 
with their respective switches. B, C and D. control the motor sides 
of any of these machines. The generator sides of the charging 
machines are shunt-wound, and in the shunt circuit is provided a 
field rheostat, having a sufficient number of points to allow for a 
very close regulation of the output of the machines at widely dif- 
ferent voltages. Of course, under normal circumstances the voltage 
of the charging machines should be that necessary to deliver the 
proper charging current to the storage battery as a whole. In the 
treatment of a defective cell, however, it is often necessary to charge 
that cell separately, and it is of great advantage if the charging 
generator may be run at the low voltage then required, without the 
use of external resistance. 

The generator leads from each of the charging machines extend 
through fuses, as shown, and through knife switches. E and F } from 
which switches the circuits of the two machines join at the point. G, 
forming the charsfinsf circuit of the batterv. One side of this 
circuit passes through the switch of an over and under load circuit- 
breaker to the negative pole of the battery, while the other side 
passes through a coil of a polarized relay to the positive or grounded 
pole of the battery. 

The functions of the circuit-breaker and of the relay are as fol- 
lows: Current passing from either of the charging machines to 
the negative pole of the batterv passes through the heavy wire coil, 



POWER PLANTS FOR COMMON BATTERY SYSTEMS. 549 

h, of the circuit-breaker, and if this current rises above a certain 
predetermined value the electro magnet, of which this coil forms 
a part, attracts its armature and trips the circuit-braker switch, so 
as to open the circuit, preventing any damage which might arise 
from an excess of current from the machines. The circuit-breaker 
has another winding, IV, of comparatively high resistance and 
greater number of turns, which winding is in the local circuit of a 
polarized relay, /. When the local circuit of the polarized relay is 
closed, therefore, the coil, h f , takes current from the charging leads 
and operates its armature to trip the circuit-breaker in exactly the 
same manner as when the heavy wire coil, h, operated. The polar- 
ized relay, /, is provided with a heavy winding, placed in series in 
that side of the charging circuit which connects with the grounded 
pole of the battery. The charging current flowing from the ma- 
chines to the battery passes through this polarized relay in such a 
direction as to hold the local circuit of this relay open. If, however, 
through any cause whatever, the voltage of the charging machines 
becomes lower than that of the storage battery, current will tend to 
back up from the storage battery through the armature and field 
of the charging generator and run it as a motor. This current 
would pass through the coil of the polarized relay in the opposite 
direction from the normal, and would therefore cause it to close the 
local circuit and open the circuit-breaker by the action of the coil, h'. 

By the means so far discussed, the batteries may be charged to 
any degree desired, and at the same time any damage due to the 
excess charging current, or to a reversal of current in the charging 
circuit, is guarded against. 

The control of ringing machine No. I is the same, as far as its 
motor side is concerned, as that of either of the charging machines. 
On the shaft of the ringing machine are provided three collector or 
commutator rings, a, b and c. Rings a and b are continuous 
throughout their circumference, but ring c is split, one-half of it 
being of insulating material and the other half of conducting ma- 
terial. Separate brushes bear against rings a and b, while two such 
brushes bear against ring c, these latter brushes being placed [8o° 
apart. There are therefore four brushes on these three collector 
rings, and from these extend four wires, a', b', c' and c" These 
wires extend to the contacts on one side of the four-lever, double- 
throw switch, K. The leads a' and b' carry alternating current 
used in work on single-party lines. The lead a' may also be used 
in connection with c' or c" , to deliver negative pulsating current or 



550 AMERICAN TELEPHONE PRACTICE. 

positive pulsating current, as required. The blades of the four- 
lever switch, K, are connected with a four-conductor cable of Xo. 
14 rubber-covered wire, from which the distribution of ringing cur- 
rent is made to each operator's position in the office. 

The armature shaft of the ringing machine carries two other 
disks, d and e, against which brushes bear, which are connected 
by wires, d' and c' , to the left-hand contacts of the double-pole, 
double-throw knife switch, L, from one of the blades of which ex- 
tends a wire through the primary coil, p, of an induction coil, and 
through an impedance coil, R, to the grounded pole of the storage 
battery. From the other blade of the switch, L, extends a wire 
which passes through a five-ampere fuse to the discharge lead from 
the ungrounded side of the battery. The contact, d', upon the ring- 
ing machine shaft is continuous as to its conductivity, while the 
contact wheel, c, has its circumference divided up by interposed 
strips of insulating material into a number of segments. The con- 
ducting segments on the circumference of the disk, e, are in elec- 
trical connection with the wheel, d, and therefore the rotation of the 
ringing machine serves to alternately break and make the circuit 
between the leads, d' and c' ', as many times during each revolution 
as there are segments in the wheel, e. As a result, when the switch 
lever, L, is thrown to the left, and ringing machine Xo. 1 is running, 
a pulsating current flows from the storage battery through the 
contact maker on the ringing machine shaft and through the pri- 
mary, p, of the induction coil, thus inducing in the secondary wind- 
ing s, of the induction coil, a periodic electromotive force for the 
production of what is known as a "tone'' used in various forms of 
signaling in the exchange. 

Associated also with the ringing machine shaft is another set of 
contact disks or wheels, f, g and i, which revolve with varying rates 
of speed. The brushes of these are connected by wires, f, g' and /'', 
to the left-hand contacts of the triple-lever, double-throw switch, M. 
The secondary, ^, of the induction coil is connected between the 
middle lever of the switch, M, and one conductor, s', of the three- 
wire cable leading to the various incoming trunk operators' posi- 
f, on the ringing machine is connected when switch, M, is thrown 
to the left, one terminal of this secondary coil is connected to the 
disk, g, the surface of which is a continuous conductor. The disk, 
f, on the ringing machine is connected when switch, M, is thrown 
to the left, with the conductor, f", also in the three-wire cable lead- 
ing to. the trunk operators' positions; and with the switch in the 



POWER PLANTS FOR COMMON BATTERY SYSTEMS. 551 

same position the disk, i, is similarly connected to the conductor, i" , 
in the same cable. The disk, f, is divided in its circumference into 
a small number of divisions and revolves slowly, and therefore when 
the conductors, s' and /', are used as a pair, in connection with a 
talking circuit, a slowly interrupted tone is heard. The tone itself 
is due to the interruption of the circuit passing through the primary 
coil, p, of the induction coil which induces current in the secondary 
winding, s, of this same coil, and the slow interruptions of the tone 
are due to the interruptions in the secondary circuit, /. It is thus 
that the busy back signal, consisting as it does of a slowly inter- 
rupted tone, is placed at the disposal of the connecting trunk ope- 
rators, the wires, s f and /", terminating in the conductors of the 
busy back jacks at each trunk operator's position. The "don't 
answer" signal is similarly placed at the disposal of the trunk ope- 
rators by leading the wires, s' and i" , to "don't-answer" jacks on 
these operators' positions. In this case, owing to the greater 
rapidity of interruptions caused by the contact wheel, i, the tone 
heard in the receiver connected with the "don't-answer" circuits is 
more rapidly interrupted than in the case of the busy back. 

It will be seen that the connections from the secondary side of 
ringing machine No. 2, including all of the disks carried on or 
operated by its shaft, are made in the same manner as 
those of ringing machine No. I, all of the leads of ringing machine 
No. 2 being led to the right-hand, instead of the left-hand, contacts 
of the switches, K, L and M. By throwing these switches to the 
right instead of to the left, the various ringing currents from these 
machines, together with the busy back and "don't-answ T er" currents, 
will be obtained from ringing machine No. 2, instead of from No. i 

Instead of being driven directly from the outside mains, as is 
ringing machine No. I, ringing machine No. 2 derives its current 
from the discharge leads of the storage battery. This motor side is 
therefore wound for the voltage available from the storage battery. 
By throwing the switch, N, the motor of ringing machine No. 2 
may be connected across the discharge leads of the storage battery, 
thus placing that machine in operation after the proper manipula- 
tion, of the starting box. The retardation coil. A\ is placed in the 
circuit leading from the storage battery to the motor of ringing 
machine No. 2, in order to prevent the fluctuations in the current 
drawn from the storage battery when this machine is running, due 
to the commutation on the motor from "throwing a noise on the 
batterv." This term, "throwing a noise on the battery," means the 



552 AMERICAN .TELEPHONE PRACTICE 

causing of the potential at the battery terminals to vary periodically 
in such manner as to render all currents delivered from the battery 
fluctuating rather than steady, which, of course, would produce an 
audible effect in all receivers drawing current from this source. 

It seems strange, at first thought, that it should be necessary to 
introduce an impedance coil in the circuit of the motor driving the 
ringing machine to prevent the fluctuations due to commutation 
from causing rapid periodic fluctuations in the battery voltage, when 
no such coils are placed in circuit with the charging generators 
which supply current to the battery. The reason for this is that man- 
ufacturers of these machines have devoted their attention to the de- 
signing of charging machines that would deliver a current prac- 
tically free from "noisy" fluctuations, while no such result has been 
attempted with regard to motors driven from batteries. If the same 
precautions were taken in designing and constructing the motor 
with this end in view as are taken in the case of charging genera- 
tors, the impedance coil would be unnecessary . 

The distribution of current from the discharge leads of the stor- 
age battery to the various parts of the exchange may be varied in 
accordance with the size of the exchange, the circuits employed 
in the switch-board and also with respect to the ideas of the engineer 
as to how great an extent the various discharge circuits should be 
subdivided. 

It will be seen that the main discharge lead, 0, from the negative 
side of the battery, terminates in a bus-bar, P, from which the 
various supply wires leading to the different parts of the switch- 
board apparatus are led. The return side of the different discharge 
circuits are connected either to bus-bars connected to the grounded 
discharge lead, Q, or to the common switch-board ground wire, S. 
Considering the distribution from the negative or ungrounded side 
of the battery, the conductor, t, is led from bus-bar, P, to the bus- 
bar, T, from which No. 14 rubber-covered wires supply current to 
the keyboard signals of the A operators, and to those of the B ope- 
rators. Two wires in each case lead to each A operator's keyboard 
and three to each B operator's keyboard. Each of these wires from 
the bus-bar, T, is connected to this bus-bar by 5-ampere fuses, these 
fuses being mounted on the face of the power board, so as to be 
readily replaced in case of burning out. In some cases it is desir- 
able to separate the leads to the keyboard signals of the A operators 
from those to the keyboard signals of the B operators, and to ac- 
complish this the bus-bar, T, may be divided as shown, the wires 



POWER PLANTS FOR COMMON BATTERY SYSTEMS. 553 

extending to the B operators' positions leading from one portion 
of it, and those to the A operators' positions to the other. These 
two sections are strapped together by a heavy enough conductor, 
usually a fuse, to carry whatever current is required, but if at any 
time it is desired to measure the current consumed by the A ope- 
rators' keyboard signals separately from those of the B operators', 
or vice versa, this can be accomplished by separating the two por- 
tions of the bus-bar, T. 

The lead, U, is tapped off from a bus-bar connected to the bus- 
bar, P, and is protected by a 5-ampere fuse. This lead, as before 
stated, supplies current to the local circuit controlled by the contact 
makers of the ringing machine shafts for the production of the 
tone. The E-shaped bus-bar, V, is connected to the bus-bar, P, by 
the lead, v, and serves to supply current to the A operators' line 
lamps, to the operators' transmitters, and to the relay racks for the 
operation of the line relays. For the first of these purposes a lead, 
w, extends from the bus-bar, V, to each A operator's position, 
where it supplies current for the operation of the line lamps. A 
separate No. 16 rubber-covered wire leads from this bus-bar 
through a 3-ampere fuse to each operator's position, and the blow- 
ing of one of these fuses, therefore, affects only the line lamps of 
that particular position. Similarly, a separate twisted pair, w' ', 
of No. 20 rubber-covered wire has one of its wires leading from the 
bus-bar, V, and the other from the bus-bar connected with the dis- 
charge lead, Q, from the positive pole of the battery to each ope- 
rator's transmitter. These are protected by i-ampere fuses. These 
wires are arranged in twisted pairs, in order to prevent the possi- 
bility of cross talk between the operators' sets. 

Another lead, w" , extends from the bus-bar, V, through a three- 
ampere fuse to each bay of relays on the relay rack. It will be 
remembered that the relays, both line and cut-off, are mounted to- 
gether on the relay rack, usually in strips of 20, and it is customary, 
for the s'ake of convenience, to have a separate battery lead servo 
each separate bay of relays, a "bay" being the space between any 
two of the uprights which serve to support the relays. One such 
bay of the relay rack is shown in the lower right-hand portion of 
the figure, and, unfortunately, somewhat resembles the ordinary dia- 
grammatic representation of a condenser. 

A heavy conductor, W, extends from the main discharge lead. 0. 
from a point back of all bus-bars, to the repeating coil rack shown 
in the lower right-hand portion of the figure. Through this is 



554 AMERICAN TELEPHONE PRACTICE. 

supplied all current to the repeating coils, and therefore all current 
used by the subscribers in talking, in the Western Electric system. 

The common night alarm relay, X, has its coil placed in the lead 
running from the ground discharge lead, Q, to all of the pilot circuits 
of the various positions. The coil of the relay, X, is, however, 
adapted to be shunted by the knife-switch, x, when it is not desired. 
to have the night alarm bell sounded. It will be seen that the night 
alarm bell, x, which is an ordinary magneto bell, is placed in the 
circuit extending from the alternating current leads of the ringing 
machines, and including the local pair of contacts of the relay, A^, 
so that whenever this relay is operated, the bell, x' , will sound, 
The night alarm switch, x, is not, as a rule, placed on the power 
board proper, but generally on one of the panels of the switch- 
board, so as to be available to the chief operator. 

The bell shown at the immediate left of the night alarm relay, X, 
is of the ordinary vibrating type adapted to use with direct current. 
It is wired between the grounded discharge lead, Q, and a contact 
strip or bus-bar placed in proximity to the bus-bar, V. Whenever 
one of the line lamp fuses on bus-bar, V, blows, it releases a spring 
which completes contact between this bus-bar and the bus-bar to 
which the bell is connected, thus causing the bell to sound. 

All the circuits and apparatus of the power-board have now been 
considered with the exception of those by which the measurements 
of the voltage and current of the different portions of the circuit 
are accomplished. As a rule, two voltmeters and one ammeter 
are placed on the power board, although instead of having two volt- 
meters a single one with a double scale is sometimes used. In the 
case shown, where two are used, the voltmeter, Y, has a scale some- 
what higher than that required to register the maximum voltage 
of the secondary sides of the charging machines, and these ma- 
chines are usually adapted when used with eleven cells of battery 
to produce a potential of about 30 volts at the generator terminals. 
Just below the voltmeter, Y, is placed a voltmeter switch having 
three pairs of stationary contacts, and that voltmeter, by the manip- 
ulation of this switch, may be connected with any one of these pairs. 
It will be seen that one pair of contacts is connected across the dis- 
charge circuit of charging machine No. 1 ; another pair across the 
discharge circuit of charging machine No. 2, and a third pair direct- 
ly across the bus-bars of the battery. By this means the voltage of 
either charging machine may be taken, when running on an open 
or closed circuit, and also that of the batterv may be measured. 



POWER. PLANTS FOR COMMON BATTERY SYSTEMS. 555 

The voltmeter, Y', is adapted to measure with great accuracy low 
voltages, the scale usually reading up to five volts, although a three- 
volt scale would be ample for the purposes required. It will be 
seen that the voltmeter switch just below this meter has a pair of 
contacts leading to the terminals of each cell of the battery, so that 
by manipulating this switch the voltage of any cell may be measured 
separately. This feature is of great value in that it enables the bat- 
tery attendant to watch the behavior of the individual cells, as well 
as of the entire battery. By means of voltmeter and hydrometer 
readings a faulty cell may be readily detected and treated. 

The ammeter, Z, should have a scale adapted to record the maxi- 
mum current that will occur in any one of the various charging or 
discharge leads on the power board. These meters are in reality 
extremely low-reading voltmeters, and serve to measure the drop 
of potential around the shunts placed in the various leads carrying 
current which it is desired to measure. By means of proper cali- 
bration of the shunt and the conductors leading from it to the volt- 
meter, the voltmeter is made to register the number of amperes 
passing through the shunt. It will be seen that the shunt, z, is 
placed directly in the main charging lead extending from the charg- 
ing generators to the negative pole of the battery. When the am- 
meter switch is thrown so as to connect the ammeter with this shunt, 
the total amount of current being delivered from the charging ma- 
chine may therefore be measured. Another shunt, z\ is placed in 
the main discharge lead, 0, and enables the voltmeter to measure 
the total current being delivered by the battery alone, or by the bat- 
tery and machines together, to the exchange. Practice differs to a 
considerable extent as to what degree means should be provided for 
measuring the various branch discharge leads, but the arrangement 
shown in this figure (397) is thought to be fairly representative of 
standard practice. A shunt, z 2 , is provided in that discharge lead 
which supplies the keyboard signals for both A and B operators, but 
if it is desired to measure the keyboard signal current of the A ope- 
rators separately from that of the B operators, this may be done by 
dividing the bus-bar, T, as already pointed out, and placing the 
shunt in circuit with each of the leads to the bar so divided. 

Another shunt, z 5 , is placed in the discharge lead, v, which sup- 
plies the bus-bar, V, from which the line lamps, the operators' trans- 
mitters and the line relays are supplied with current. Still another 
shunt, z 4 , is placed in the lead from the live side of the battery to 
the repeating coil rack, and as all current for talking purposes is fed 




556 



POWER PLANTS FOR COMMON BATTERY SYSTEMS. 557 

through the repeating coils, this shunt serves to measure the entire 
current consumed by the subscribers' lines in talking. 

Of course, it is possible to effect a greater subdivision of the cur- 
rent, as for instance, a shunt might be placed in each of the leads, 
w, w' and w" , instead of having a single shunt to measure the cur- 
rent consumed by all of these leads. There is very little advantage, 
however, to be gained by too great subdivision in this respect, as it 
merely serves to complicate matters unduly, and very often the 
current in one set of leads alone would be so small as to render its 
measurement on the ordinary ammeter provided, almost an im- 
possibility. 

Coming now to the actual construction of the power board, Fig. 
398 gives a good idea of a modern board furnished by the Kellogg 
Switch-Board and Supply Company in one of the large central 
office equipments of the Keystone Telephone Company in Phila- 
delphia. This board is of marble, and all apparatus is so mounted 
upon it as to be handled from the front of the board, while the cir- 
cuits are all accessible from behind. This and many other power 
boards are of white Italian marble, but a later and perhaps better 
practice is to make the boards of a fine quality of slate, to which an 
oil finish is given by smearing it with a heavy coat of oil, which is 
afterwards burned in. By this means a velvety black finish is im- 
parted to the slate, against which the burnished copper instrument 
and switches present a handsome appearance, so that when such a 
board is properly constructed, its appearance is not less attractive 
than that of marble. Slate of the proper quality has an advantage 
over marble in that it is less liable to be traversed by metallic streaks 
which sometimes afford conductive paths from one part of the power 
board to another, and thus cause much trouble. There is but little 
difference between the cost of the best slate and ordinary Italian 
marble. Poor slate is more likely to be defective than marble. 

The rear view of such a switch-board is shown in Fig. 399, which 
well illustrates the method of wiring and connection furnished in 
large modern telephone plants. 

A full discussion of power machinery for telephone plants, that 
is, of the charging and ringing machines, would involve most of 
the points which have to be considered in the design and construc- 
tion of dynamo electrical machinery in general. There are. how- 
ever, a few points in which the dynamos and motors used for tele- 
phone work are peculiar. The condition met with in telephone work. 
which is not found to an equal extent in any other field of electrical 




55S 



POWER PLANTS FOR COMMON BATTERY SYSTEMS. 559 

engineering, is that the current delivered by the charging generators 
or consumed by any motor deriving its current from the central bat- 
tery, must be as nearly as possible absolutely "smooth." The 
fluctuations of electromotive force and current due to commutation 
at the brushes, or to the entrance of the various armature conductors 
into, or their passage from, the field of force of the machine, should 
be eliminated to such an extent that no noise whatever will be heard 
in the talking circuits deriving currents from the terminals of the 
battery during the use of the machines. Obviously, if the voltage 
across the terminals of the storage battery were subject to rapid peri- 
odical fluctuations due to the action of the charging machines, or 
motors driven from the battery, a noise would be heard in all re- 
ceivers connected across lines fed by the battery. 

It was formerly necessary, with the then available charging ma- 




FIG. 400.— IMPEDANCE COIL. 

chines, to place a heavy impedance coil in the charging leads to re- 
duce the fluctuations in current and consequent noise in the talking- 
circuits, and even with this it was often necessary to charge only in 
such hours as the exchange was least busy, that is, at night. By 
means of the machines now available, however, this is not neces- 
sary; the machine may run in the daytime or whenever necessary. 
and the objectionable impedance coil, which, of course, consumes a 
certain amount of energy, is done away with. An impedance coil for 
use with machines not suitable for giving sufficiently smooth current 
is shown in Fig. 400. 
Probably the principal factor in the design of charging machines 



560 



AMERICAX TELEPHONE PRACTICE. 



accomplishing this result is the use of a much greater number of 
commutator bars than would be required for almost any other type 
of direct current machine. The brushes on such machines with the 
yery narrow commutator bars used cover approximately three com- 
plete segments on the commutator. The armatures are of the 
smooth core type, the winding being continuous and employing a 
greater number of conductors than would ordinarily be the case. It 
is found that with the slotted armature type of construction, wherein 
the conductors are bunched in groups between the teeth of the arma- 
ture, a sufficiently smooth current cannot be produced. Another 
important factor is that the machine should be as nearly as possible 
magnetically and electrically balanced, which can only be attained 
by great care in its mechanical design and construction. 




FIG. 401.— MAGNETO POWER RIXGER. 



The Holtzer-Cabot Company, of Brookline, Mass., which makes 
an excellent line of telephone charging and ringing machines, em- 
ploys diamond-shaped pole faces on their charging machine pole 
pieces, the object of this being to cause the armature conductors to 
enter into and retire from the magnetic field as gradually as possi- 
ble, so as to prevent any sudden fluctuation in the voltage. 

As stated earlier in this chapter, the same care in the design of 
motors used in the battery ringing machines as is exercised in the 
case of modern charging machines would make those machines 
capable of drawing a current from the battery sufficiently steady to 
prevent noise without the use of impedance coils in the discharge 



POWER PLANTS FOR COMMON BATTERY SYSTEMS. 



561 



lead to the motor. This is a matter of subsidiary importance, how- 
ever, as the battery ringing machine is in most cases used only as 
a reserve, and manufacturers have not yet seen fit to construct 
special small motors for this purpose. 

Considering now some specific examples of telephone charging 
and ringing machines, Fig. 401 shows a five-bar magneto generator 
for ringing purposes in small exchanges, adapted to be driven by 
belt from any available source of power. A similar generator belt 
connected to a direct current motor is shown in Fig. 402. 

In more recent practice the magneto generator in all but the 
smallest exchanges is discarded, alternating current generators with 




FIG. 402.— MAGNETO GENERATOR BELTED TO DIRECT-CURRENT MOTOR. 



fields electro magnetically excited being used. Where low voltage 
(not over 220) direct current is available, the dynamotor is often 
used for ringing purposes. In this the motor and the generator 
windings occupy the same armature core, which revolves in a field 
common to both windings. Such a machine is shown in Fig. 403. 
this being of approximately one-sixth horse-power. For higher di- 
rect current voltages, the motor generator type of ringing machine 
is best adapted to furnishing ringing current in telephone ex- 
changes. 

The reason why it is not well to use dynamotors when only a high 
potential direct current is available for driving it is that in such 
cases there is always a liability that the insulation between the pri- 
mary and secondary windings of the machine will break down under 

36 



562 AMERICAN TELEPHONE PRACTICE. 

the stress of the greater electromotive force, thus impairing the ac- 
tion of the machines. Of course where the motor generator is used 
the two armatures are entirely separate, and this danger does not 
exist. It may be said in general that the dynamotor is not an 
efficient machine for any but ringing purposes in a telephone ex- 
change under any conditions. When used for charging much 
trouble is had due to inefficient regulation. The fact that a single 
field serves for both the generator and the motor side of the machine 
makes it impossible to regulate the two sides independently. 

Inasmuch as one or the other of the ringing machines are, in a 
modern exchange, driven during twenty-four hours of each day, 




FIG. 403.— SMALL DYNAMOTOR FOR RINGER. 

these machines become the most available for driving the busy-back 
and tone-test attachment already referred to in connection with Fig. 
397. A dynamotor for ringing purposes adapted to be driven by 
direct current is shown in Fig. 404, and this has permanently at- 
tached to it a busy-back of obvious construction. 

Until recently trouble has been experienced in applying alter- 
nating current motors to the direct driving of ringing machines. 
The reason for this was that all alternating motors available ran at 
such high speed that when direct-connected to ringing machines 
the frequency of the alternating current generated thereby was too 
great for properly actuating the bells. As a result of this direct-con- 
nected machines were not used in such cases, the ringing generator 



POWER PLANTS FOR COMMON BATTERY SYSTEMS. 563 

being belted to an alternating current motor in such manner as to 
run at lower speed. A ringing machine equipped with busy-back 




FIG. 404.— SMALL DYNAMOTOR WITH BUSY-BACK ATTACHMENT. 

and don't-answer attachment and belted to an alternating current 
motor is shown in Fig. 405, this being a set used by the Stromberg- 
Carlson Company in one of their exchanges. Recently the Holtzer- 




FIG. 405.— RINGING MACHINE WITH BUSY-BACK ATTACHMENT BELTED 
TO ALTERNATING-CURRENT MOTOR. 

Cabot Company have perfected a low speed alternating current 
motor built specially for this purpose, so that the ringing machine 



564 



AMERICAN TELEPHONE PRACTICE. 



will run at the proper speed to give the required number of cycles 
per minute. 

It has frequently been found expedient to attach busy-back devices 
to old ringing machines, originally furnished without them, and for 
this purpose the Holtzer-Cabot Company has built a busy-back out- 
fit as a separate unit so arranged that it may be fitted to the shaft of 
almost any ringing machine. One of these devices is shown in 
Fig. 406. 

A modern charging machine is shown in Fig. 407, this consisting 
of a four-pole direct current motor of standard construction direct- 
connected to a specially constructed telephone charging generator. 




FIG. 406.— SEPARATE BUSY-BACK ATTACHMENT. 



As is seen, the two machines are mounted on a heavy castiron sub- 
base common to both so as to maintain a permanent alignment of 
the bearings. In Fig. 408 a similar charging set is shown, the gen- 
erator being direct-connected to an alternating current motor, thus 
making alternating current mains available for power purposes. 

As a rule, the power machines, including both charging and ring- 
ing generators and their motors are mounted in close proximity to 
the power switch-board, which latter includes all devices for control- 
ling the action of the various machines. It has been the practice 
of some companies to mount the ringing and charging machines on 
a table or bench of wood with perhaps an iron framework. Such an 
arrangement is shown in Fig. 409, where two special telephone 
motor generators and two ringing motor generators are mounted 
compactly on a table. The primary of one of the ringing machines 



POWER PLANTS FOR COMMON BATTERY SYSTEMS. 565 

is wound for the external power circuit and that of the other for the 
battery circuit. 

The mounting of the power machines on a wooden table with in- 




FIG. 407. -CHARGING MOTOR GENERATOR-DIRECT-CURRENT MOTOR. 

closing panels of wood, as shown, is faulty in one respect; the table 
and its inclosing panels serve to enhance any noise that may be made 
by the machines, and for obvious reasons they should run as quietly 




FIG. 408.— CHARGING MOTOR GENERATOR— ALTERNATING-CURRENT 

MOTOR. 



as possible. A better practice is to build up a pier of brick from the 
floor of the power room and to provide this with a heavy slate top. 
This tends to greatly reduce the amount oi noise of the machines. 



566 



AMERICAN TELEPHONE PRACTICE. 



and this may be still further decreased by resting the sub-base of 
each machine on a heavy piece of felt. 

Such a brick pier and with a slate top is shown in Fig. 410, which 
represents a power table equipment installed by the Kellogg Switch- 
board and Supply Company in one of the large exchanges of the 




FIG. 409.— POWER TABLE— WOOD BASE. 



Keystone Telephone Company of Philadelphia. All of these ma- 
chines are made by the Holtzer-Cabot Company. The two charging 
machines are of y\ horse-power each on their motor sides, and are 
identical in construction with those shown in Fig. 407. In this plant 
three ringing machines were used owing to requirements made by 
the Keystone Company, two of them being adapted to use on the 
outside power mains (one as a reserve for the other) and one to run 
from the current of the storage battery. 



POWER PLANTS FOR COMMON BATTERY SYSTEMS. 



567 



It is customary in most modern work to build the pier for 
the power table of white enamel bricks highly glazed and to provide 
a top of heavy slate with a burnt-oil finish. By this means a con- 
struction exceedingly handsome, durable and effective is secured* 

The range through which it is sometimes necessary to regulate 
the output of charging machines is, as has been stated, very wide. 
It is desirable to be able to reduce the voltage from that necessary 
to charge the whole battery to that necessary to charge a single cell. 
The field rheostats of ordinary commercial practice in other lines of 
electrical work do not have a sufficient range of resistance for this 
purpose, nor a sufficient number of steps between its maximum and 
its minimum resistances to enable the regulation to be effected with 




FIG. 410.— POWER TABLE— BRICK AND SLATE BASE. 



great enough nicety. The remedy for this is more contacts on the 
rheostat and therefore more steps. Frequently, however, a sufficient 
number of contacts for the desired result cannot be placed on the 
rheostat face without making it of prohibitory size. The Holtzer- 
Cabot Company has again come to the rescue of telephone men in 
this difficulty by producing a rheostat consisting of two discs with a 
certain number, say fifteen, contacts each, each disc being provided 
with a separately movable rheostat arm or wiper adapted to move 
over all the contacts on its disc. The resistance coils on the front 
disc are of sufficient resistance to give the required range of action of 
the machine, while the coils on the rear disc are of such resistance 
that the entire fifteen of them equal the resistance of one coil on the 



568 



AMERICAN TELEPHONE PRACTICE. 



front disc. The field resistance may then be adjusted roughly on the 
front disc, and then by manipulating the lever of the rear disc incre- 
ments may be added until the desired resistance is obtained. 

In the double-faced rheostat shown in Fig. 411, a total of 225 steps 
is obtained on a rheostat of the diameter of an ordinary fifteen-point 
rheostat. 

The same general rules that apply to the care of dynamo electrical 
machinery in general will apply to charging and ringing machines 
in telephone work, but on account of the nicety with which these 
machines are made and adjusted, if of the proper type, an even 
greater amount of attention should be paid to their care. In tele- 
phone work special stress should be laid on the necessity of main- 
taining the commutator brushes in good order, as carelessness in 
this matter is one of the most prolific causes of disturbing noises 




FIG. 411.— HOLTZER-CABOT RHEOSTAT. 



caused by machines in the telephone lines. The brushes when 
properly set on a properly designed machine, will not cause noise in 
the lines even when no choke coils are placed in the charging leads. 
It has been stated that the brush should be wide enough to bear on 
about three complete segments on the commutator, and they should 
be fitted to the commutator in such manner that the entire sur- 
face of the brush makes contact with the commutator surface. 

A good way to fit a brush after the brush-holder is properly 
placed is to lay fine sand-paper on the commutator, rough side out, 
and then draw it under and brush in the direction of the rotation of 
the armature, the brush being pressed against the sand-paper by the 
spring of the brush-holder. During the first rough adjustment of 



POWER PLANTS FOR COMMON BATTERY SYSTEMS. 569 

the brush in this manner, no harm will be done if the sand-paper 
is pulled back and forth under the brush, and to facilitate the secur- 
ing of the approximately proper surface of the brush quickly, extra 
pressure may be placed on the brush-holder. In putting the final 
touches on the wearing surface of the brush, however, no additional 
pressure than that of the spring in the .brush-holder should be 
applied, and the direction of the motion of the sand-paper should be 
the same as that of the motion of the armature, great care being 
taken to keep the sand-paper closely pressed against the surface 
of the commutator. 

In multipolar machines the brushes should be equally spaced 
around the commutator. 

The number of commutator bars between each two adjacent sets 
of brushes should be equal. After the brushes are all properly 
set, the rocker upon which all of them are mounted may be moved 
in one direction or another until a position is reached in which 
no sparking at the brushes will occur on all loading of the machines, 
from no load to full load. It is sometimes found that after careful 
adjustment all brushes will behave properly except one, and when 
this is the case, it should be carefully resurfaced, and if it still 
sparks, may be adjusted back and forth separately, leaving the 
other brushes as they are. This condition, however, in a properly 
designed machine, and with uniform and suitable brushes, is not 
liable to occur. 

A properly working commutator presents a rather dark, glossy 
appearance. A commutator in this condition seems to be glazed, 
the general color being considerably darker than that of freshly 
cut copper. The appearance, therefore, of bright streaks, which 
are sometimes rather rough, indicates that some or all of the 
brushes are cutting, and this should be remedied. 

The best lubricant for the commutator is probably vaseline, but 
it should be very sparingly used. It should be applied with a small 
clean rag into which vaseline has been thoroughly, but sparingly 
soaked, so that no lumps of vaseline will come off on the com- 
mutator. 

As a rule, telephone central offices, when first installed, are 
equipped for only a portion of the number of lines that it is 
thought it will be eventually called upon to serve. The switch-board 
and most of the auxiliary apparatus are equipped to a sufficient 
extent to meet the present requirements, and arrangements arc 
made whereby they may be added to in future as the number of lines 



570 AMERICAN TELEPHOXE PRACTICE. 

increase. This is not the case, however, with most of the power 
plant apparatus, which must be installed at the outset with sufficient 
capacity to serve the ultimate number of lines which it is thought 
will eventually enter the office. The only exception in this regard, 
in connection with the power plant, is the storage battery, as it is 
possible and feasible to provide only enough plates to serve for 
present requirements, room enough being left in the tanks, how- 
ever, for the addition of subsequent plates to meet the demands 
of the extended service. In most power plants, therefore, the full 
load current output of the charging machines is in excess of the 
normal charging, rate of the batten'. 

The greatest efficiency in the operation of the power machines 
is obtained when the machines are run at approximately full load,, 
and therefore it is more economical to charge during the period 
of the day when the traffic is greatest. At this time there is a 
heavy discharge from the battery, and the current delivered to the 
battery through the charging leads may be greatly in excess of 
the normal charging rate of the battery. Obviously, however, at 
such times the effective charging current which the battery is 
absorbing, is the difference between the current in the main charging 
leads and that in the main discharge leads; in other words, it is the 
difference between the ammeter reading around the shunt, s, and that 
around the shunt, z\ Fig. 397. 

In case of sudden demands on the service by virtue of which 
traffic becomes so great as t© exceed the maximum discharge rate 
of the battery, the charging machines may be called into service to 
help the battery out, in which case the battery and the charging 
machine are both delivering current to the discharge leads. 

The length of the daily run is determined by the traffic and 
should be continued each time until the batteries indicate full charge. 
The charging rate should be limited either by the capacity of the 
battery or of the machine, which ever has the smallest capacity. On 
account of the fact before mentioned, it is usually true that the 
battery has less capacity than the machines. It is desirable that 
the batteries should be fully charged each day, and preferably that 
this full charge should be reached at a time after the period of 
heaviest traffic in the day. This is true because in the case of a 
complete disability of all charging machines, the battery would act 
as a reserve and should be able to run the plant for twenty-four 
hours without additional charge. It is also true because the wel- 
fare of the battery itself depends on its being fully charged at 



POWER PLANTS FOR COMMON BATTERY SYSTEMS. 571 

frequent intervals, and on its remaining discharged as short a time 
as possible. 

It is obvious that the ringing machine, which derives its power 
from the main power leads, should be used regularly, and that the 
one which runs from the battery current should be used only as 
a reserve. 



CHAPTER XXIX. 
STORAGE BATTERIES. 

If two plates of lead are immersed in a weak solution of sul- 
phuric acid, no difference of potential will be established between 
them, because the acid, if it acts on them at all, does so to an 
equal extent on each plate. If now an electric current, as from 
a battery or a direct-current dynamo, is sent through the two 
plates and the solution between them, a redistribution of mate- 
rials will take place in the cell. The electrolyte will be decomposed, 
the oxygen in it forming, with the plate to which the positive terminal 
of the charging force is connected, lead peroxide ; while hydrogen is 
liberated at the plate to which the negative terminal is connected. 
On disconnecting the source of current, the cell, which was before 
incapable of producing a difference of potential, is found able to 
drive a current through a circuit formed by connecting its poles 
together by a wire or any other conductor. The combination has 
become a voltaic couple. 

The cell, consisting of two lead plates in a solution of sulphuric 
acid, was devised by Gaston Plante, and is the prototype of all 
storage batteries or accumulators that have come into general use 
up to the present time. Nearly all commercial cells, of which there 
are many, have the plates coated with, or in close mechanical con- 
tact with, some compound of lead, rich in oxygen. This is changed 
by the charging current into lead peroxide on the positive plate, 
and to spongy lead on the negative. In this condition the cells will 
give an electromotive force of slightly over 2 volts, the pressure 
remaining nearly constant during the greater portion of the time 
while the cell is being discharged through some external circuit. The 
direction of the current flowing from the cell while discharging 
is always opposite to that of the charging current. A charged 
storage cell behaves exactly like a primary battery, but it has the 
advantage that after being discharged it can again be made useful 
without the addition of any material whatever, by merely sending a 
current through it in the proper direction. 

In all but the smallest storage cells more than two plates are 
used, all the positive plates being connected by a heavy strip of 

572 



STORAGE BATTERIES. 



573 



lead, and likewise all the negative plates by another similar strip. 
There is usually one more of the negative than of the positive 
plates, the arrangement being such that the plates are alternately 
positive and negative. 

The extremely low internal resistance of storage batteries, and 
the fact that their voltage is high (2 + volts per cell) and constant 
and that they are not subject to polarization, make them, all 
things considered, the ideal source of current for telephone work. 
They are much more economical in operation than any form of 
primary cell, inasmuch as there is practically no consumption 
whatever of the materials in the cell itself, it depending of course 




FIG. 412.— ELEVEN-PLATE CHLORIDE CELL. 

for its energy on some outside source. Their ease of manipula- 
tion and general cleanliness and reliability are also strong points 
in their favor. They have long been used for supplying the 
operators' transmitters in large central offices, but the recent devel- 
opments leading to the almost universal adoption of the various 
common battery systems have vastly increased their field of use- 
fulness, since they are now called upon to furnish current for 
subscribers as well as operators. 

Among the several good storage batteries on the market, the 
chloride accumulator made bv the Electric Storasre Battery 



574 



AMERICAN TELEPHONE PRACTICE. 



Company of Philadelphia may be mentioned first on account of 
is being more widely used than any other. In this the negative 
plate is composed of a number of small square blocks of spongy 
lead held together by a grid of lead with a small amount of 
antimony added for hardness. The blocks are made by fusing 
together zinc chloride and lead chloride, after which they are 
placed in a mold and the grid cast around them under pressure. 
Afterwards by an electro-chemical process all traces of zinc are 
removed, leaving the composition of the blocks pure spongy 
lead. The positive plate consists of a lead-antimony grid in 
which circular holes are molded. These holes are filled with buttons 




FIG. 413.-NINETEEN-PLATE CHLORIDE CELL 'WITH WOODEN TANK. 



made by rolling up a crimped lead ribbon in the form of a spiral, 
of a size to fit tightly in the holes. The plates are then treated 
electro-chemically in order to form the proper oxide from this ribbon. 
The positive and negative plates so formed present a large sur- 
face to the electrolyte for it to act upon, the negative plates on 
account of the porous, spongy, lead blocks, and the positive plates 
on account of the crimped lead ribbon. The plates are held apart 
by long hard-rubber washers hung over each end of each negative 
element. These are placed in a vertical position to prevent forming 
a shelf upon which loose particles from the plates might lodge and 
form a short-circuit. These cells have amply demonstrated their 
adaptability to telephone use by long service in this and other fields. 



STORAGE BATTERIES. 



575 



The electrolyte for these cells is a mixture of sulphuric acid 
and water in the proportion of about five parts of water to one of 
acid. The proper specific gravity of this mixture, as specified by 




FIG. 414.— BATTERY PLANT. 



the manufacturers, is from 1180 to 1190, as indicated on an ordinary 
hydrometer. 

In Fig. 412, the general appearance of one of these cells having 
six negative and five positive plates is seen. Fig. 413 shows a larger 
•cell having ten negative and nine positive plates mounted in a 



576 



AMERICAS TELEPHONE PRACTICE. 



lead-lined wooden tank. Lead-lined wooden tanks are preferable 
to glass jars in large batteries, on account of the liability of glass 
to breakage. In small capacity batteries, glass cells are used in most 
cases, one of their advantages being that they allow of free inspec- 
tion of the plates more than would lead-lined tanks of small size. 

In Fig. 414 is shown a portion of two batteries of chloride accu- 
mulators as installed in one of the large common battery exchanges 
of the Kellogg Switch-Board and Supply Company. These batteries 
have eleven cells each, thus giving approximately twenty-four volts 
each. They are used for supplying all current for talking and sig- 
naling in the entire exchange, except that used for ringing the bells 





FIG. 415.— DETAILS OF AMERICAN CELL. 



of the subscribers, which is, of course, alternating or pulsating in 
character. 

Another type of storage battery used to a less extent is that 
made by the American Battery Company of Chicago. In this bat- 
tery the plate is made from a solid sheet of pure rolled lead deeply 
grooved on both sides so as to leave projecting ribs, one-twentieth of 
an inch apart, -affording a very large surface for the electrolyte to 
act upon. The form of a single plate is quite clearly shown in Fig. 
415. The ribs are slightly upturned in order to better retain the 
active material carried in the grooves between them. The active 
material in the plate is electro-chemically formed in a strongly oxi- 
dizing solution, filling the grooves and covering the surface of the 
ribs with an adherent peroxide of lead coating. The positive and 
negative plates are alike in construction, the active element, spongy 



STORAGE BATTERIES. 577 

lead, in the negative, being reduced from the peroxide of lead of 
the positive. Hard-rubber insulators, shown in Fig. 415, serve to 
separate the plates as well as to hold them clear of the bottom of 
containing jars. The insulators are held rigidly in place, being 
firmly keyed to the plates and surrounded by heavy rubber bands 
as shown in Figs. 415 and 416. 

Instructions for Installation, Care and Mainte- 
nance of Storage Batteries. 

Setting Up and Connecting. 
The elements, before being placed in the glass jars, should be 
examined carefully, and any foreign substance which may have 



FIG. 416.— COMPLETE NINE-PLATE AMERICAN CELL. 

lodged between the plates should be removed. See that the hard- 
rubber insulators are in position and that the positive plates do not in 
any way touch the negatives. After the elements are placed in the 
jars, connect the terminals — the positive of one element to the 
negative of the next and so on. 

The best method of connecting the cells permanently is to burn 
the terminal strips together, or when it is not practicable to do this 
solder them together, using a hot iron or soldering torch, first 
scraping bright the surface to be soldered. For a soldering flux 
it is best to use ordinary pure tallow. Under no circumstances 
should muriatic acid or soldering salts lie used. For temporary 
connections the connectors usually sent with the elements will an- 
37 



578 AMERICAN TELEPHONE PRACTICE. 

swer. To prevent corrosion, these must be painted with some pro- 
tective paint and well covered with okonite tape. 

The best method to thoroughly insulate the cells is to place each 
one on a separate wooden base painted with good insulating paint, 
this base resting on glass or porcelain insulators ; or to place them 
on shelves or tables covered with sheet glass. Care should be taken 
to place cells in a cool, well-ventilated place, where each cell is 
readily accessible, so that elements can be removed when necessary 
with the least possible trouble. 

When lead-lined tanks are used, the cells should be placed on the 
floor if the available space permits. 

Lead-Burning. 

As stated under the preceding heading, the best way to per- 
manently connect the lugs or terminal strips of the separate cells 



LARGE FLAME TIP 




v \\* 

\ 

\ 

FIG. 417.— BURNER FOR LEAD BURNING. 

is to burn them together. This process is not difficult to carry out 
if well understood, but of course the beginner should first experiment 
on lead strips that do not form the terminals of storage cells. 

The best flame to use for lead burning is the hydrogen flame, 
as this can be used without flux and in general produces cleaner 
work. A hydrogen generator is necessary for this, this usually 
consisting of two lead chambers, one above the other and con- 
nected by a lead pipe. The pipe leads from a point near the bottom 
of the lower chamber, out of the top of this chamber, and into the 
bottom of the upper one. Equal parts of zinc and sulphuric acid 
are put in the bottom chamber. The acid in attacking the zinc forms 
zinc sulphate and liberates hydrogen gas. As the gas is generated 



STORAGE BATTERIES. 



579 



the acid solution is forced up into the upper chamber, thus main- 
taining a fairly constant pressure, which is usually from 6 to 8 
pounds per square inch. A rubber tube serves to lead the gas from 
the top of the lower chamber to the burner tip. 

The acid used for this purpose should be free from arsenic and 
most other impurities. Arsenic will make a white deposit on the 
lead, and thus may cause a poor connection. 

The burner used for this work is usually of a form shown in 
Fig. 417, having two leading-in tubes, one for air and one for gas. 
These are provided with several tips having different sized holes 
for obtaining different sizes of flames. Air must be supplied to 
the burner by an ordinary form of foot-bellows or by any other 
available means. 

Lead burning may often be successfully done by using ordinary 




POSITIVE TERYINAL 




NEGATIVE TERMINAL 



L 



7 



u 



FIG. 418. 



SHEET IRON TROUGH. 

-METHOD OF BURNING SMALL TERMINALS. 



illuminating gas instead of hydrogen, but this depends largely on 
the quality of the gas. Natural gas is sometimes used, but this is 
likely to prove unsuccessful. In any case the work is somewhat more 
difficult to perform than when hydrogen is used. 

The method of procedure will depend somewhat on the form 
of terminals to be jointed. On comparatively small cells the ter- 
minals are usually of the form shown in Figs. 412 and 416. With 
these, the terminals should be bent down and beveled off as shown in 
Fig. 418, and the surfaces to be jointed should be scraped perfectly 
clean and bright. A scraper made by securing a triangular piece of 
steel with sharp edges to a suitable handle, as shown in Fig. 410. 
is very convenient for this purpose. A small sheet-iron trough, as 
shown in the lower portion of Fig. 418, and having an interior 
cross-section equal to the cross-section of the terminals, should be 



580 



AMERICAN TELEPHONE PRACTICE 



slipped over the two lugs from beneath and secured in place by a 
wooden clamp or in any other convenient manner. A chamber is 
thus formed into which the melted solder may be run as described 
later. 

If the battery is of large size, it will usually have a number of 
lugs, which will be joined to a cross bus-bar when two cells are con- 
nected. This form is clearly shown in Figs. 413 and 414. When 
burning such lugs and bus-bars together, a pair of lead burning 
tongs made of iron, and of a form shown in Fig. 420, should be 
used. The front surfaces of these are beveled to an angle accur- 
ately corresponding to that of the bus-bar, so that they will fit snugly 
against the bus-bar when in place. The inner surfaces of these 
tongs should be parallel when opened just wide enough to fit the 
lug on the cell. The lug after being properly beveled and scraped 
is brought into position almost touching the bus-bar, the surface 
of which has also been scraped, after which the tongs are applied in 




SHARP EDGE 




FIG. 419.— SCRAPER FOR LEAD TERMINALS. 



such manner as to form a chamber in which to run the melted solder. 

Having proceeded thus far according to either of the methods 
described, the blow-torch, using either hydrogen or illuminating 
gas, may be brought into play. The solder should be pure lead. 
No flux will be needed with the hydrogen, but tallow will be re- 
quired with the other gas. The solder should be melted off the 
stick and allowed to drop into the chamber, and at the same time 
the surfaces to be joined should be kept just at the melting point 
by a judicious application of the flame. The chamber may thus 
gradually be filled, and if all goes well the solder will unite perfectly 
with the lead surfaces, thus making a continuous piece of metal. 

The melting point of lead is not far from 6io° F., and in burning 
the lugs to a bus-bar of a large cell great care must be taken not to 
entirely melt the lugs before the bus-bar is sufficiently hot — it re- 
quiring, of course, more heat than the lugs. In the flame of the 
burner there will be noticed an outer bluish-red and rather scatter- 



STORAGE BATTERIES. 



581 



ing flame, and an inner flame blue and well defined. The best re- 
sults are usually obtained by holding the point of this inner blut 
flame on the surface to be burned. In burning lugs it is usually 
better to use a comparatively large flame, as shown in the upper 
portion of Fig. 417. In burning seams, however, the small flame 
shown in the lower portion of that figure is more desirable. 

After the metal has thoroughly set, the tongs or clamps may be 
removed and the joint trimmed up a little if necessary for appear- 
ance. It is always well to leave the joint of slightly larger cross- 
section than the lugs. 

Electrolyte. 

After the cells are connected up, and the terminals of the battery 
led to the switch-board so that the charging current is ready to be 




FIG. 420.— METHOD OF BURNING LUGS TO BUS-BARS. 



turned on, the electrolyte may be added. It is important that this 
should not be done before. 

The electrolyte, consisting of a mixture of pure sulphuric acid 
and water, preferably distilled, should indicate a specific gravity of 
1 190 on the ordinary specific gravity hydrometer or 23 Baume 
scale. This solution should be mixed in stone jars in about the 
proportion of five parts water to one of concentrated sulphuric acid, 
by volume, pouring the acid slowly into the water. It is very dan- 
gerous to pour the water into the acid, and one cannot be too care- 
ful on this point. The electrolyte becomes hot after mixing, and 
it should be allowed to cool for at least four hours before using. 
Under no circumstances allow the glass jars to be used for mixing 



582 AMERICAN TELEPHONE PRACTICE. 

purposes. River and well water usually contain impurities and 
should be avoided, as the least quantity of chlorine or ammoniacal 
salts present in the electrolyte will seriously affect the life of the 
plates. 

Charging. 

Great care must in all cases be taken that the batteries are so con- 
nected that the charging current passes through them in the proper 
direction. The positive pole of the battery must be connected to 
the positive pole of the generator. The positive pole of the gen- 
erator leads may be determined by the use of a direct-current volt 
or ammeter. A simple test, and the most convenient one when 
neither ammeter or voltmeter are available, for determining which 
is the positive pole of any source of current is to dip wires leading 
from both terminals into a small vessel containing slightly acidu- 
lated water. Bubbles of gas will be given off from each wire, but 
at a very much higher rate from the wire leading to the negative 
pole than from that leading to the positive. The poles of the charg- 
ing dynamo should always be determined with absolute certainty 
before connection is made to the terminals of the storage battery, 
for a reversal in the connections is very likely to ruin the battery. 

As soon as possible after the electrolyte has been poured into the 
glass jars, seeing that the plates are covered with at least one-half 
inch of the fluid, the elements should receive their first charge. It 
is well to begin at about half the normal rate of charging, but after 
making sure that all is going well the charging may proceed at the 
normal rate, which is always given by the manufacturers, but which 
may be found by dividing the normal rated capacity of the cells in 
ampere hours by eight. Thus for a cell the rated capacity of which 
is 400 ampere hours the normal charging rate would be 50 amperes. 
The manufacturers of the American cell advocate the continuance 
of the first charge for 30 hours, while the chloride people advocate 
20 hours. 

During the last hours of the first charge the solution will appear 
to boil. The specific gravity of the electrolyte which fell much be- 
low 1200 shortly after it was first poured into the cells should have 
risen to 1225 by this time. If it is higher than this, water should be 
added until the solution indicates uniformly 1225; on the other hand, 
add dilute sulphuric acid if specific gravity doer not show 1225. A 
lower charging current than the normal may be used if it is not 
convenient to obtain that rate, but the time of charging must be 



STORAGE BATTERIES. 583 

continued proportionately longer. A 10 per cent, or even 20 per 
cent, overcharge will not injure the elements in any way; excessive 
overcharging, however, if practiced often, seriously shortens their 
life. Even though for any reason the battery is only slightly used, 
it should, nevertheless, be charged fully about once in two weeks. 

Determination of Amount of Charge. 

By means of an accurate low-reading voltmeter the state of charge 
may be ascertained quite closely, providing that the same rate of 
charge be employed regularly. In charging at the normal rate 
when the pressure at the cell terminals indicate 2.5 volts it may be 
assumed that the cell is charged within 90 per cent, of its rated 
capacity, so that when this point is reached the charging operation 
may be discontinued. On the other hand, when the voltage falls 
as low as 1.8 or 1.9 the cell may be considered discharged, and re- 
charging should begin as soon as possible. 

The hydrometer may be used with a fair degree of accuracy to 
show the condition of a cell, inasmuch as the density of the solution 
varies between full charge and discharge. As this variation is a 
function of the ratio of the area of plate surface to the volume of 
electrolyte, it is difficult to state a definite rule, inasmuch as there 
is no constant relation between plate surface and electrolyte in the 
various sizes of cells. The specific gravity of the electrolyte may 
be noted at full charge, — i. e., when the voltage is 2.5. — and that 
indication made a basis for the determination of future full charge. 
Another reading of the specific gravity made when the voltage is 
1.9 per cell may be made a basis for future determination of the dis- 
charged condition of the cell. A little careful observation will soon 
enable the one in charge of battery to determine approximately its 
condition by means of the hydrometer alone. 

Batteries may be charged from a direct-current, incandescent 
lighting or power circuit, depending on the charging current 
wanted. The resistance used may consist of lamps in parallel or 
iron-wire coils conveniently arranged as in the ordinary motor- 
regulating rheostat. Where the best efficiency is desired it is ad- 
visable to use a motor generator, the generator end being wound 
to suit the number of cells to be charged. This method is the only 
practical one for 500-volt direct or alternating-current circuits. In 
charging from any source of electricity, an automatic under and 
overload circuit-breaker should be used, so as to prevent current 



584 AMERICAN TELEPHONE PRACTICE, 

from the battery passing back over the line in the event of a shut- 
down at the power house. 

Discharging. 

At least 75 per cent, of the energy in a good storage cell is ob- 
tained before the voltage falls below 1.9 volt. It is advisable not to 
discharge beyond this point ordinarily, although a discharge to 1.8 
is allowable. A cell may be considered discharged at 1.8 volt. 

Never under any circumstances completely exhaust the battery. 
If a battery is allowed to stand discharged for a period exceeding 
two or three days, the capacity of the cells may be found on subse- 
quent discharge to be materially lessened, due to conversion of the 
lead oxides into inert sulphate of lead. 

Replacing of Electrolyte. 

The boiling of the electrolyte when charging causes a fine spray 
to arise, by which some of the fluid is carried off, so that diluted 
acid must sometimes be added to replace it. All spilled solution 
should be replaced by solution of like density. Evaporation loss 
should be replaced by the addition of water only. The electrolyte 
must be maintained at all times over the tops of the plates and 
each cell individually inspected at least once a week, when the 
strength of the electrolyte should be tested, and if the density is below 
1200 when other indications point to the full charge of the cell, 
dilute acid should be added until the density reaches that figure. 
Never under any circumstances pour pure acid into a cell in order 
to bring the electrolyte up to the proper density. A portion of the 
old fluid should be siphoned off and fresh acid in a diluted form 
added. When water is added to replace loss due to evaporation, 
it should be added at the bottom of the cell either by a glass or 
rubber tube or syringe. 

Defective Cells. 

If for any reason one or more of the cells should act strangely, 
— that is, show a marked difference in color, voltage, or specific 
gravity indications from the others, — they should be examined at 
once and the cause of the trouble ascertained. Should a nail or other 
foreign substance, or any material scaling from the plates them- 
selves, lodge between the plates, it will cause a short-circuit, and 
this will be indicated by low voltage and low specific gravity, and 
should be at once removed. Its most probable cause is the lodging 
between the plates of some foreign article, but it may also be due 



STORAGE BATTERIES. 585 

to the depth of sediment in the bottom of the cells reaching the 
bottom of the plates. If the short-circuit is due to a foreign body, 
it should be removed ; if to a loosened portion of the plates, it may be 
forced to the bottom of the cell ; if to sediment, the cell should be 
cleaned out. A strip of hard rubber that will go between the plates 
is convenient for use in this work. If the short-circuit is removed 
promptly, no harm will result ; on the other hand, if the cause of a 
short-circuit is allowed to remain, the plates may be seriously 
affected, injured possibly beyond repair. 

It is important to use only glass, rubber, or wood in reaching 
into the cells. Metals will be attacked by the electrolyte, and thus 
impair its purity. Moreover, they are likely to short-circuit the 
cells and thus damage the plates. 

Treatment of Short-Circuited Cell. 

A cell that has suffered a short-circuit will need more than its 
usual amount of charge after the trouble has been removed. This 
may be obtained in various ways. First, by overcharging the whole 
battery, a bad practice if, done too frequently, but it may be occa- 
sionally resorted to without evil effects. Second, by cutting out of 
circuit the cell in trouble on one or two discharges and replacing 
it on the charges. This can be most conveniently done on small 
cells with bolted connections, but not very well with cells perma- 
nently connected. Third, by giving the cell in trouble an individual 
charge during the discharge of the battery. Current for this may 
be obtained from the regular charging dynamo, working through a 
resistance if necessary. The third method is the best one, and is 
easily carried out. 

Color of Plates. 

The color of the plate is a valuable indication as to the condition 
of the cells. 

The positive plate should have a dark brown, velvety appear- 
ance. Any lightness in color indicates insufficient charging. The 
negatives should have a clear bluish lead or light slate color. 

Cells that have not been sufficiently charged for a continued 
period, or that have been left standing uncharged too long, are 
liable to what is termed sulphating. This is indicated by a white 
deposit on the plates due to the formation of lead sulphate which 
is insoluble in the electrolyte, and is deleterious to the action of 
the cells. When it occurs between the lead of the plate and the 



586 AMERICAN TELEPHONE PRACTICE. 

active material it is liable to loosen the latter, causing it to drop off, 
or, if it remains on the plate to increase the ^resistance of the cell. 
The remedy for sulphating is to continue the charging current at 
about half normal value for several hours after the cells give indica- 
tion of being fully charged. 

A whitish deposit sometimes appears on the surface of the 
plates in spots, even when the cells have not been subject to under- 
charging. This, the manufacturers claim, need not give alarm. It 
usually disappears after the battery has been in service some months. 
It is probably a mild form of sulphating, occurring only on the 
outer surface of the active material. 

Taking a Battery Out of Service. 

Charge the battery fully at a rate not higher than the normal. 
Siphon off the acid (which may be used again) into convenient 
receptacles, preferably thoroughly clean carboys, and immediately 
refill each cell with water. Then discharge the battery at the normal 
rate down to less than one volt per cell. 

The elements should then be removed from the water and allowed 
to dry. They may then be replaced in the empty tanks or jars, pre- 
viously dried and cleaned, or stored in any dry place. 



CHAPTER XXX. 
PROTECTIVE DEVICES. 

Protecting telephone apparatus from the damaging effects of 
currents other than those which properly belong on telephone lines, 
and guarding the users of such apparatus against accidents and 
their property from fire, caused by such currents, has claimed the 
attention of telephone engineers in no small degree. 

The problems involved have been numerous and varied because 
of the widely different conditions which may, and often do, arise 
to make protection necessary. To-day the protective apparatus used 
in a telephone exchange forms an important part of the entire 
equipment, as on it depends not only the safety of the lives of the 
operators and users, but of the physical property of the telephone 
company and that of its customers. 

The devices used for this purpose are broadly termed "arresters," 
this being a shortening of the term "lightning arresters," which 
was universally used when the currents due to lightning were 
practically the only extraneous currents to be dealt with. The term 
arrester now has significance with respect to all kinds of damag- 
ing currents, whether they be due to lightning, or to any other 
source, natural or artificial. 

The advent of the street railway and electric lighting and other 
systems in the allied fields of electrical engineering, and of the 
common battery system now largely used in the telephone industry 
itself, has placed before the telephone engineer more complex con- 
ditions than were present in the days when the old "saw tooth" 
arrester afforded what was thought to be adequate protection for 
telephone apparatus. 

There are broadly three elements against which apparatus must 
be protected : lightning, high tension currents, such as may be caused 
to flow on a telephone line by a cross with electric light or power 
wires ; and sneak currents, which are currents too small to do in- 
stantaneous damage, but which, if they persist, may, by the accu- 
mulation of heat, cause damage to the apparatus. Of course, only 
the first two causes need be considered in regard to the danger to 
human life. 

587 



588 



AMERICAN TELEPHONE PRACTICE. 



The lightning arresters found on nearly all telephone instruments 
up to within a few years ago, were of the saw tooth type, and usually 
of the general form shown in Fig. 421. This was an adaptation of 
the lightning arrester frequently used on single telegraph lines at 
intermediate stations where the two wires, which looped into the 
office, terminated respectively in the binding posts on the plates, 
Aand B, between which posts the local instruments were also looped. 
The plate, C, was connected directly with earth. By inserting a 
conducting plug in the hole, e, the local instrument would be short- 
circuited and the line left connected through. If the plug were 
inserted in the hole, g, that end of the line terminating in plate A 
would be grounded, while the local instruments would be connected 
in circuit with that end of the line terminating in plate B. Simi- 
larly, inserting the plug in hole, /, would ground the end of the line 
connected with the plate B, and leave the instruments connected 




FIG. 421.— SAW-TOOTH ARRESTER. 



in operative relation with the end of the line terminating in the 
plate A. 

When applied to a telephone instrument, or more properly, to a 
sub-station equipment, the plates A and B form the terminals of the 
instrument and also the terminals of the line, the plate C, as before, 
being grounded. When telephone instruments are connected in series 
in a party line, as was the common practice in the early days, 
either end of the line could be cut off and grounded, leaving the 
instrument in operative relation with the other end in exactly the 
same manner as described in connection with the telegraph line. 
This formed a convenient means of cutting out a defective end of 
the line when comunication was desired with the other end. On a 
telegraph line, or on a series telephone line this device, therefore, 
served admirably as a switch, in addition to its functions as a light- 
ning arrester. The latter functions may be described as follows : 
When the line was subjected to a high potential charge of electricity, 



PROTECTIVE DEVICES. 589 

this charge would seek the most available path to earth, and this 
would usually be across the air gap between the ground plate and 
the plates A or B. The current, as a rule, would take this path rather 
than pass through the high impedance coils within the telephone. 

In lightning discharges, and, in fact, high tension discharges from 
other causes across an air gap, the current is often of an oscillatory 
nature of inconceivably high frequency, and, therefore, the imped- 
ance of the coil even though it consists of but a few turns, pre- 
sents more of a barrier than would an air gap of considerable length. 
It is on this principle that the action of the air gap arresters are 
based. It is frequently true, however, that a shorter air gap than 
that afforded by the arrester is found between the wire of a coil and 
its core, and from the core to the ground. For this reason, telephone 
apparatus which has grounded cores is, as a rule, defective, and it 
may be stated further that when the core is grounded it is better to 
have the line wire connected with the outside layer of winding rather 
than the inside layer. 

The old saw tooth arrester, besides being inefficient, has serious 
drawbacks, from a practical standpoint. The subscriber would often, 
on the approach of a thunder storm, short-circuit his instrument 
by inserting a plug in the hole, e, or connect it directly to earth by 
inserting a plug in one or both of the holes, f and g. This would 
afford the instrument as perfect protection as could be given, but he 
would frquently forget to remove the plug after the storm, there- 
fore leaving his instrument out of service ; and if the line used bridg- 
ing instruments the insertion of the plug in the hole, e, would also 
leave most or all of the other instruments on the line out of ser- 
vice ; also, if it were a series party line, the insertion of the plug 
in one of the ground plate holes, would sever the connection between 
the two ends of the line. 

A lightning arrester much more efficient than the one shown in 
Fig. 421, but based on the same general principles, consists of a 
pair of carbon blocks held apart by a thin disc of mica. One form of 
this which has come into wide use is shown in Fig. 422, which has 
two pairs of blocks, one for each side of a metallic circuit line. One 
block of each pair is connected to the line wire it is to protect, while 
the other block of each pair rests on a metal ground plate. These 
pairs of blocks are usually arranged as in this figure, to slip between 
rather strong springs and the ground plate, the springs forming the 
line terminals. By this arrangement the blocks may be easily re- 
moved when desired. The thin mica strip between the pairs of blocks 



590 AMERICAN TELEPHONE PRACTICE. 

is perforated so as to allow a small air gap between the blocks, over 
which the high potential charge may jump with much greater ease 
than between the comparatively widely separated plates of the old 
saw-tooth arrester. The length of the air gap as determined by the 
thickness of the separating mica strip usually varies from .005 
inches to .007 inches. Carbon block arresters of this general type 
are now almost universally used for protection against lightning, 
and against all currents such as might produce potentials of 300 
volts or over between the line and ground. With a distance of .005 
of an inch between the carbon blocks a pressure of 300 volts across 
the blocks will break down the insulation of the air gap between 
them. 

This kind of an arrester, of course, operates by grounding the 
line either temporarily or permanently; and in the case of a light- 
ning discharge, which persists for only a minute period of time, no 



FIG. 422.— CARBON BLOCK ARRESTER. 

other protection is necessary. However, a high potential current due 
to a cross with a high tension wire is likely to persist after being 
grounded, and thus cause a very large current to flow over the con- 
ductor so grounded, which may be injurious to the line wire or cable 
itself. For this reason it becomes desirable to provide some means 
for opening the circuit after it has been grounded in case the current 
allowed to flow by the grounding is of sufficient magnitude to become 
dangerous. The most simple means of accomplishing this consists 
of a fuse wire of limited carrying capacity. Such a wire is usually 
made of some low-fusing metal having comparatively low conduct- 
ing power, such as lead, the object being to make it the weakest 
link, electrically speaking, in the chain of conductors. 

In Fig. 423 is shown in diagrammatic form the combination of 
fuse and carbon arrester as they are associated together on telephone 



PROTECTIVE DEVICES. 591 

lines, with a view to first protecting the instrument from the cur- 
rent to which a high tension cross might subject it, and second, to 
protecting the line from the current which would flow after the 
line was grounded at the carbon arrester. In this, A represents the 
air gap between the two plates of the carbon arrester, and F the 
fuse wire, it being obvious that if a high tension cross occurs on the 
line the arcing across the air gap will protect the telephone instru- 
ment, after which the current flowing over the now grounded line 
will, if it becomes dangerous, cause the fuse, F, to blow thus entirely 
opening the line and preventing all flow of current. 

The general principles governing high tension and strong current 
protection have now been dealt with, but none of the means so far 
pointed out are sufficient to cope with the sneak current, against 
which an entirely different means of protection must be employed. 

Sneak currents may be caused by a low potential cross some- 
where on the line, or by comparatively high potential cross through 

^r-To Telephone^ , ^r^ n li™ w£re 



T 



Jflea tension 

J i Jirc hyhilny 
I Dynamo^* 



FIG. 423.-DIAGRAM OF FUSE AND CARBON ARRESTER. 

a very high resistance. Cases have been known where they were 
caused entirely by induction from some wire carrying high potential 
alternating current in the operation of a lighting or power circuit. 
In common battery work still another very common cause of sneak 
currents is due to the grounding of one of the telephone wires or 
the crossing of two wires, in which cases, even though the lines 
are not subjected to any outside electromotive force, the current flow- 
ing from the common battery may, in some systems, persist to such 
an extent as to cause ultimate damage to the apparatus. 

The most simple means of protecting against these small currents 
is to employ a fuse wire of very small carrying capacity. These 
wires may be made sufficiently small to fuse when subjected to the 
passage of a current of \ ampere, and fuses supposed to blow at J 
ampere have been widely marketed. These small capacity fuses 
are usually mounted on mica strips to which they are secured 
usually bv varnish or shellac, the ends of the strip being provided 



592 AMERICAN TELEPHONE PRACTICE. 

with some form of terminal to which connection may readily be 
made without subjecting the fuse proper to mechanical injury. 
Such a fuse is shown in Fig. 424, the terminals in this case being of 
heavy tin-foil wrapped around the ends of the mica strip, the fuse 
wire being electrically securely connected to the terminals at each 
end. Such forms of fuses are adapted to slip between spring clips 
mounted on insulating blocks and provided with binding posts for 
securing the terminals of the circuit to be protected. These fuses 
play a very important part in telephone and telegraph work, par- 
ticularly in small installations. Frequently, in combination with 
carbon block arresters, they form the only protection afforded to 
the line, the apparatus and the user. They are not, however, effi- 
cient forms of sneak current arresters, for the reason that they can- 
not be depended upon to open the circuit when traversed by current 
of predetermined strength. It is frequently found that a fuse sup- 
posed to blow on being traversed, by a certain current will safely 
carrv four times that. Another disadvantage is that they are, on 




FIG. 424.— POSTAL-TYPE FUSE. 

account of their small size, very frail and liable to mechanical injury. 

On account of these objections an entirely different type of sneak 
current arrester has come into use, depending for operation on the 
thermal effect of the current. In these the heat generated by the 
passage of the current in a small body, usually a coil of low con- 
ductivity, closely confined, melts not the coil itself, but a particle of 
low-fusing solder in close proximity to it. These devices are com- 
monly termed heat coils. 

The idea of the heat coil was, so far as the writer is aware, first 
introduced by Mr. H. V. Hayes, of Boston. His device consisted of 
a flat coil, through the center of which projected a pin fixed in place 
by a drop of low-fusing solder. Against this pin rested a spring 
which bore against the pin with considerable force tending to push 
it through the coil. When a sufficient current passed through the 
coil to generate enough heat to melt the solder, the pin would give 
way and allow the spring to come in contact with the ground plate 
which would throw a dead ground on the line. 

Another pioneer in this line was Mr. F. B. Cook, of Chicago, 



PROTECTIVE DEVICES. 



593 



whose arresters are now widely known and used. The construc- 
tion of one of Cook's heat coils is shown in Fig. 425. In this c is 
a hard rubber cylinder having a tapped hole extending throughout 
its length, into one end of which is screwed a flanged brass piece, a y 



- — ^r 




■^^y^^^^^^^^y 




FIG. 425.— HEAT COIL. 

and into the other a brass plug, b. A flanged piece, d, carries a wire 
rod, e, which extends through a hole in the plug, b, and is soldered 
therein by a low-fusing solder. In the chamber, /, which surrounds 
the enlarged portion of the plug, b, is contained a coil of German 
silver wire wrapped about this plug. One end of this wire ter- 
minates in the plug itself, to which it is soldered, and the other ex- 
tends through the small hole, g, in the hard rubber block and is 
soldered to the piece, a. 

The heat coil is slipped between the terminal springs as is shown 



d 



a 



A 



PL 

FIG. 426.-HEAT COIL AND HOLDER. 

in Fig. 426, the springs, A and D, being the terminals respectively 
of the line wire and the wire leading to the instrument to be pro- 
tected. The circuit may be traced from the line through the spring 
A, thence to the flanged head, a, through the German silver wire 



594 AMERICAN TELEPHONE PRACTICE. 

in the hole, g (Fig. 425), through the heat coil proper, and to the 
plug, b, and other coil head, d, to the spring D. The springs A and 
D are held together by the coil, their tension being such as to tend 
to pull the coil apart. When, therefore, a current of sufficient 
strength to melt the solder passes through the coil, the pin, c, at- 
tached to the head, d, is released, allowing this head to be withdrawn 
from the plug, b, thereby opening the coil and allowing the spring 
A to come in contact with the ground spring, B, thereby shunting 
the line current to ground. 

All of the general types of protective devices that are in common 
use have now been discussed, but before passing to the considera- 
tion of these devices in detail, it will be well to consider what is at 
present regarded the proper degree of protection for telephone ap- 
paratus and lines. This may be discussed by reference to the dia- 
gram of Fig. 427. In this figure is shown a line extending from 
the central office to a subscriber's instrument, this line passing 
directly from the office through a section of underground cable, 
thence to a section of overhead cable, and thence through bare 
wires on poles to the subscriber's premises. The underground 
cable, it is understood, runs directly into the central office, being 
terminated either in potheads or their equivalents, or directly on 
the line side of the distributing frame, so that there is practically no 
danger of a cross occurring between one of the telephone wires and 
a power wire at any point between the central office and the outer 
end of the underground cable. 

At the point in the office where the conductor emerges from the 
cable, is provided a combined carbon and sneak current arrester, 
the carbon arrester being shown diagrammatically at A and the heat 
coil of the sneak current arrester at B. This latter coil is so ar- 
ranged that when released it will allow the line spring, b, to make 
contact with the ground connection, thus grounding the line and 
either opening the circuit to the switch-board, or merely grounding 
it while leaving it closed. The latter has the advantage that the 
ground applied by the heat coil spring will light the line lamp, if the 
wire in trouble leads to the line relay, and a notice of the action 
will so be given ; this is not true if the line wire is merely opened. 
At the outer end of the underground cable at the point, C, where 
connection is made with the overhead cable, a fuse should be placed, 
but this connection is frequently made with no protective device 
whatever. A fuse should always be placed at D, the junction point 
of the overhead cable and the bare wire, and still another fuse at E, 



PROTECTIVE DEVICES. 595 

the point where the line wire enters the subscriber's premises. Some*- 
times a carbon lightning arrester is also added at D, as shown in 
dotted lines. Within the subscriber's premises, and frequently form- 
ing a part of the telephone equipment, is placed the carbon arrester, 
F, and in some instances a heat coil is also used at this point. 

Most engineers prefer to place a fuse between the underground 
and overhead cable, as at the point, C. It is undoubtedly an addi- 
tional protection and might prevent serious loss where a heavy cross 
existed between the sheath of the overhead cable and a high tension 
wire. Such a cross might easily cause the current to arc to 
the sheath, and, melting a hole in this, to the conductors within 
the overhead cable, in which case the fuse at C would probably 
save the conductor in the underground cable. If a cross with 
a potential of over 300 volts occurs on any of the conductors, 
a flow of current will probably take place through the car- 
bon arrester at A, or F, or both, and the consequent flow due to 



T o s witrch board, 



heat 
coil 




& 



Overhead, cable 



L^ bare wire r . To telephone 



>C 



E 



f 



Vnderaround. cable 

FIG. 427.-DIAGRAM OF COMPLETE PROTECTION FOR TELEPHONE LINE. 

the low resistance path thus established would cause sufficient cur- 
rent to flow to blow one or more fuses at C, D or E. If, on the 
other hand, a comparatively low potential cross existed not of high 
enough potential to cause the carbon arrester at A or F to act. and 
not causing a current of sufficient strength to flow to blow the fuses 
at C, D or E, the heat coil, B, would receive the current and would 
operate, throwing a dead ground on the line, and thus effectually 
prevent any current entering the exchange apparatus. The low 
resistance path thus leading to the ground would, if the current 
were of sufficient strength to affect the conductors in the cable, 
cause the blowing of one or more of the fuses. 

The placing of combined heat coils and carbon arresters at the 
exchange of the line fuse at C or D and E, and of the carbon arrester 
ati 7 , represents well established practice. The sensibility of the vari- 
ous protective devices and also the questions as to whether a fuse. 



596 AMERICAN TELEPHONE PRACTICE. 

or carbon arrester or both should be placed at C, whether a carbon 
arrester should be added to the fuse at D, and whether a heat coil 
should also be added to the protective devices at the subscriber's 
station are all subject to discussion. 

There is a difference of opinion among exchange engineers as to 
whether it is desirable or not to secure absolute protection. Pro- 
tection that is to all intents and purposes absolute may be secured 
by putting in all of the devices mentioned and by making them suffi- 
ciently sensitive. Under this condition, however, the heat coils 
may be frequently operated by the normal currents of the exchange ; 
again, if the line fuses are made of too low carrying capacity very 
slight currents which might injure nothing would also cause their 
operation. If the gaps in the carbon arresters are made too small, 
the liability of short-circuiting the line to ground by the accidental 
contact between the carbons, or the presence of a small amount of 
carbon dust, would exist. 

The blowing of a heat coil or the short-circuiting of the carbon 
arrester at the exchange is not a matter of much importance, be- 
cause it is an easy matter to quickly replace them, men being always 
present in the exchange for that purpose. However, all such un- 
wished for occurrences at the office arrester, even though remedied 
within a few moments, are likely to cause interruptions in the ser- 
vice which, from the subscriber's standpoint, is not desirable. The 
blowing of a fuse or the operation of a carbon arrester or heat coil 
at some point in the outside construction or at the subscriber's 
station is, however, much worse, as the interruption to the service 
is of longer duration and the cost of repairing is much greater. 

It is seen, therefore, that there is such a thing as overdoing pro- 
tection, as too perfect a system, considered from the standpoint of 
protection alone, brings about an increased cost for maintenance 
and frequent interruptions of service. For this reason a middle 
course is usually pursued, aiming to give sufficient protection to the 
apparatus and property to prevent the possibility of any far-reaching 
disastrous results, at the same time keeping in mind the minimizing 
of the cost of maintenance and of service interruption. 

With these general ideas in view the various questions as to best 
protection in the typical case shown in Fig. 427 may be discussed . 
That the central office protection should consist of a heat coil and 
carbon arrester is a matter concerning which there is little dispute; 
although there are those who maintain that a line when entirely 
underground from the central office to the subscriber's premises 



PROTECTIVE DEVICES. 597 

has no need of any protection whatever. In the case shown in Fig. 
427, however, where a portion of the line circuit is aerial, the force 
of this contention is removed. 

The degree of sensitiveness of the heat coil cannot be specified 
for all cases, as the apparatus of some switch-board systems is very 
much more susceptible to damage by excessive currents than others. 
In other words, currents which would be excessive for the electro 
magnets of some systems would be carried readily without danger 
of harm by those of others. It may be said in this connection that 
the best switch-board designers are making the coils of electro 
magnets, as far as possible, self-protecting; so that they would not 
be damaged by any current due to the voltage of the central office 
battery which might flow through them, even on a short circuit. 

The electro magnets of some switch-boards are wound with such 
fine wire as to render the following requirements of the heat coil 
necessary: That it shall carry .1 ampere indefinitely and operate 
on .2 ampere within 5 minutes. To meet these requirements the 
heat coil is generally wound to about 20 ohms resistance. In other 
cases the requirements are that the coil shall stand .2 ampere in- 
definitely and that it must operate on .25 ampere within 3 minutes, 
and that the resistance of the coil shall not be more than y\ ohms. 
These latter figures are now perhaps the most commonly adopted, 
and with such a coil when the current is in excess of \ of an ampere 
but less than .4 of an ampere the coil will operate in from 1 to ij 
minutes; and with currents in excess of .4 of an ampere they are 
operated practically instantaneously. 

In a letter from Mr. Frank B. Cook to the writer on this subject, 
Mr. Cook says: "My own personal opinion is that in view of the 
constantly increasing number of high tension circuits (10,000 to 
30,000 volts) that all telephone apparatus should be made to stand 
.3 ampere for an indefinite period, and to operate on, say, .35 ampere 
within 2 minutes. This would admit of the heat coil having a re- 
sistance of 5 ohms or less. This is simply my* opinion and not what 
is practiced." 

The air-gap between the arrester blocks at A, Fig. 427, is now 
fairly well standardized at .005 inch, and with such an air-gap and 
with carbon of ordinary grade the insulation between the blocks 
will break down when subjected to a tension of about 300 volts. 

The question as to whether a fuse should be placed at C, Fig. 
427, or not, that is, at the point where an underground cable joins 
an aerial cable, is a mooted one, but it is thought that the chance for 



598 AMERICAN TELEPHONE PRACTICE. 

trouble, due to omitting the fuse at this point, is far too great to 
warrant the risk of omitting it. In other words, practice has proved 
in general that the occasional loss due to damaged cables caused by 
the omission of fuses at these points is of more importance than 
would be the maintenance of fuses at all points where the under- 
ground and aerial cables join. 

The propriety of putting fuses in all wires in the cable at the point, 
D , where the overhead cable joins the bare wire or where an under- 
ground cable is connected to a bare wire without the intervention of 
an aerial cable, is very questionable. There are, however, many who 
prefer to use carbon arresters also at this point, and many exchanges 
are so equipped. It is sometimes desirable to put carbon arresters 
at the point, D, to protect the cable against lightning discharges from 
long lines. Where this is done, the fuse is necessary in order to pro- 
tect the pole equipment from currents which may flow, due to arcing 
at the carbon and consequent fires therefrom. As the office of car- 
bons at D is to arrest lightning and not stray currents of lower poten- 
tial, the separation of carbons at this point should be made about 
i -io inch or greater. 

Mr. Cook's views on this subject are as follows: "I have noticed 
for some time past that the use of the combined fuse and carbon 
arrester is increasing at a very rapid rate, and now I think a ma- 
jority of good construction companies are following the plan of 
using the combined fuse and arrester. I also notice that there are 
many complaints of damage to cables at points where the fuse 
alone is used, and in many instances, well-managed telephone com- 
panies who were formerly using simply a fuse for protection to their 
cables, have replaced the fuses with the combined fuse and carbon 
arrester, all of which indicates that they have learned by experience 
that fuses are not good lightning arresters, and this is true, of course, 
regardless of their carrying capacity." The carrying capacity of 
the tubular fuses used in outside construction work varies from 
about i to 7 amperes' Probably in the various Bell companies the 
/-ampere fuse is most commonly used, while most of the Independent 
companies are using somewhat smaller carrying capacity, averaging 
perhaps 2\ or 3 amperes. Where, however, a carbon arrester is used 
in connection with the line fuse the usual carrying capacity of the 
fuse is 5 amperes. It would perhaps be safe to say that as far as 
standardization has progressed, the various Bell companies use the 
7-ampere on practically all outside construction work, while the 
Independent companies use fuses adapted to blow 2-J or 3 amperes. 



PROTECTIVE DEVICES. 599 

Probably all telephone engineers agree that a fuse should be placed 
at the points where the line wire enters the subscriber's premises, 
as at E } in Fig. 427. Most complete telephones are now also 
equipped with fairly efficient forms of carbon lightning arresters, 
these latter forming integral parts of the telephone as produced. The 
combination of the fuse and the arrester, therefore, forms a very 
efficient protection for the instrument, the premises, and in fact for 
the line conductor itself. Some companies, however, add a heat coil 
to the protective device at the subscriber's premises, but this is 
thought to be not in accordance with the best practice. The cost of 
burnt-out telephones which would have been saved by the presence 
of a heat coil would probably prove much less than the cost of 
time and labor of replacing heat coils at the subscriber's stations. 

One of the difficulties met in solving the problems of adequate 
and economical protection may be realized when it is considered 
that in any common battery exchange there must be a certain flow 
of current over the lines for the operation of the signals, and for 
furnishing talking current to the subscribers. The heat coils, of 
which there are at least two in each line circuit, must be so made 
as not to be affected by these currents, even though they some- 
times approach very close to what is considered the dangerous limit, 
yet they must operate with certainty on slightly heavier current, par- 
ticularly if it persists for any length of time. 

The question has often arisen relative to the efficiency of carbon 
as a sparking or discharging surface for lightning, and as to its 
possessing qualities for this purpose superior to those offered by 
metals. The reason for its alleged superiority for this work is prob- 
ably the fact that in carbon small particles of material on its rough- 
ened surface often become detached and lie in a loose state, 
in which they may possess the qualities of the coherer, main- 
taining an open circuit until cohered by the lightning dis- 
charge. After storms much trouble is often experienced from line 
grounds due to the presence of this dust, it being necessary to remove 
these grounds before service is restored. Schemes have been tried 
by which the carbon may be prevented from giving off this dust ; one 
of these being to soak the carbon block in resin or paraffin. The 
object desired was thus undoubtedly attained, but unquestionably 
at the sacrifice of the sensitiveness of the sparking surface. The 
use of certain metals as a substitute for carbon in the usual carbon 
block arresters is being experimented on, and developments in these 
will be of general interest. 



600 AMERICAN TELEPHOXE PRACTICE. 

Coming now to a discussion of the actual devices and apparatus 
used in the protection of telephone equipments and lines, the matter 
of fuses will be first considered. One of the usual forms of sneak 
current fuses is that shown in Fig. 424, this being known as the 
Western Union type. Another common fuse differing only in the 
style of its terminals is shown in Fig. 428, this being known as 
the Postal type. 

The salient points of difference between the Western Union and 
the Postal fuses is that the former is adapted to insertion between 
spring clips, while the latter is secured in place on its clips 
by cross screws extending through the slots in its ends. These 
fuses are, as a rule, adapted to inside construction only. If, there- 
fore, they are placed outside, they should be inclosed in weather- 
proof boxes of substantial character. 

Such fuses are often mounted in conjunction with carbon arresters 
so as to form protection against both low potential and high poten- 
tial currents, the theory of such operation and connection with the 




FIG. 428.— WESTERN UNION FUSE. 

line circuit being the same as described in connection with Fig. 
423. Such a combination adapted to a single line wire is shown 
in Fig. 429, which, as will be seen, employs the Postal type of fuse. 
The two carbon blocks of this figure are adapted to be held in the 
spring clips shown at the upper portion of the porcelain base, the 
mica washer serving to keep them from actual electrical contact. 
For metallic circuit lines these devices are usually made up in pairs, 
mounted on porcelain blocks, there being one fuse and carbon ar- 
rester for each side of the line. In Fig. 430 such a device for a 
metallic circuit adapted to use with Western Union fuses is shown, 
while that in Fig. 431 employs the same combination adapted to 
Postal type fuses.- In Fig. 432 is shown one of these combined 
carbon and fuse arresters applied to a line circuit entering an ordi- 
nary magneto-telephone. This practice is used in many country 
exchanges, and as a cheap form of protection is fairly efficient. 

For use on outside line construction in such portions of the line 
as are shown at C, D and E, in Fig. 427, a form of fuse gener- 



PROTECTIVE DEVICES. 



601 



ally known as the tubular line fuse has come into wide use. One of 
these is shown in sectional and in perspective view in Fig. 433. In 
this a long fuse wire is inclosed in a tube of either wood or 



Carbon 
Bloc>\ 



CROUND--- 
Connection 



LlN£ A 

Connection; 




Instrument 
Connection 



FIG. 429.— COMBINED FUSE AND CARBON ARRESTER SINGLE 
CONDUCTOR. 

fibre, this tube being closed at its ends by heavy brass terminals 
provided with ready means for attaching the line wires. The fuse 
wire within the tube is secured to the terminal at each end by solder, 
the fuse wire extending entirely through the terminal at each end, 








\x 



FIGS. 430 AND 431.— POSTAL AND WESTERN UNION TYPES OF 
CARBON AND FUSE ARRESTERS. 



so that its ends may be accessible for soldering. The fuse is made 
in this general form for two reasons, one of which is to give a 
substantial case in which the comparatively delicate fuse may be 



602 



AMERICAN TELEPHONE PRACTICE. 



held safe from mechanical injury. The second reason is that ex- 
periment has shown that the incasing of a fuse in a tube, whether it 
is hermetically sealed or not, tends to prevent the arcing of a heavy 
current across the terminals after the fuse has blown. In case the 
fuse is hermetically sealed, the forming of gas prevents the arc from 
being maintained, and sometimes the casing explodes as a result. 
When there is an opening in the casing, the expansion within due to 




FIG. 432. -FUSE AND CARBON ARRESTER AS APPLIED TO MAGNETO 

TELEPHONE 



heat and gas that is formed, tends in passing through the opening, 
to blow whatever arc is formed. 

The method of clamping the wire at each end of the fuse, shown 
in Fig. 433, is obvious, one end of the line wire being clamped be- 
tween the parallel jaws at the right hand end of the cut, and the 
other end between the brass terminal block and the inside nut at 
the left hand end of the cut In each case the outside nut serves as 
a lock nut. This particular fuse is one made by the American 
Electric Fuse Company, of Chicago. It is particularly adaptable 
to clamping directly on the line wire outside of the subscriber's 
premises as at the point, E, in Fig. 427, the parallel jaws serving to 
grip the line wire usually at or near the insulator, while the wire 



PROTECTIVE DEVICES. 



603 



leading to the subscriber's premises is clamped by the nuts at the 
other end. 

When used on a cable pole, as for instance at the point, D, in Fig. 
427, these fuses are usually placed in boxes and mounted in banks 
for economy of room. In this case the line wires leading from the 
cable and from the bare wires are permanently soldered to spring 






FIG. 433.-TUBULAR FUSE. 

clips between which the fuse proper is adapted to slip. Such a 
construction is shown in Fig. 435, the fuses in this case being held 
in place between the clips by clamping nuts at their ends, as clearly 
shown, Fig. 434 giving detail of the unmounted fuse. Fig. 436 
shows a similar arrangement, but in this case each fuse has associ- 
ated with it a carbon arrester, thus affording additional protection 
against high-tension charges. 

A complete equipment for subscribers' stations protection consist- 
ing of tubular fuses, heat coils and carbons all mounted on a single 
porcelain block in a very compact and convenient form, is shown in 
Fig. 437. This device was designed by Mr. F. B. Cook to meet 




FIG. 434.— TUBULAR FUSE. 



the requirements of those who prefer this complete protection at 
the subscriber's premises. 

It has been said that most complete telephone sets are now 
equipped with carbon block arresters, and one of these, as manufac- 
tured by the Kellogg Switch-Board and Supply Company, is shown 
in Fig. 438. The method of associating this with the line circuit 
and with the circuit of the instruments which terminates in the 



604 



AMERICAN TELEPHONE PRACTICE. 



line binding posts is clearly shown in Fig. 439. This arrester con- 
sists merely of two brass discs of approximately semi-circular form, 
one being permanently connected to each of the line binding 
posts of the instrument. Over these fit a perforated mica washer of 
circular form, and over this is clamped, by means of a centrally 




FIG. 435.— BANK OF TUBULAR FUSES. 



disposed screw set, a carbon plate, covering both the semi-circular 
brass plates. The screw holding this carbon in place is permanently 
connected to the ground binding post, these connections being shown 
in Fig. 439. 

The lightning arrester usually furnished by the Stromberg-Carl- 
son Company on their telephone sets is shown in Fig. 440, in which 
the binding, posts shown are those of the telephone instrument. 

Coming now to the question of heat coils and the associated devices 
used for protection at the central office, it may be said that the 




FIG. 436.— BANK OF TUBULAR FUSES WITH CARBON ARRESTERS. 

coil shown in Fig. 425 is typical of standard practice in heat coil 
construction, it being the type almost universally used until a very 
recent time. 

As two heat coils and two carbon arresters are used in connection 
with each line circuit in a modern central office, economy of space 



PROTECTIVE DEVICES. 



605 



and in the amount of wiring between them, has brought about the 
association of the carbon arrester with the heat coil device in such 
manner as to form virtually one piece of apparatus. These combined 
carbon and heat coil arresters are, in order to further economize 
space, usually mounted in groups or banks of twenty pairs, on long 




pppo*"^ 



FIG. 437.— PROTECTIVE APPARATUS FOR SUBSCRIBERS' STATIONS. 

heavy strips of cold rolled iron, a single strip containing from five 
to twenty banks, each bank containing twenty pairs; each 
strip therefore contains^ from ioo to 400 pairs of arresters, 
according to the requirements of the available space and of 
the cable distribution within the office. A view of a single 
pair of office arresters as designed by Cook, and manufactured by 
himself and by the Sterling Electric Company, is shown in Fig. 
441, this view showing a horizontal section through a part of the 





FIG. 438.— KELLOGG CARBON ARRESTER FOR TELEPHONE SETS. 



arrester strip, heat coils and carbons. The heat coil in the left hand 
portion of this figure is shown in proper relation to its springs, 
as in normal use. That at the right hand portion, however, shows 
the heat coil after it has been operated, thus allowing the springs 
to separate, opening the line circuit and closing it to ground. The 



606 



AMERICAN TELEPHONE PRACTICE. 



circuit through these arresters may be traced by following the 
arrows marked a and b, these arrows leading from the wires ex- 
tending to the switch-board to the wires extending to the outside 
line. Following arrow a, it will be seen that the circuit extends from 




GROUP 



FIG. 439.— CONNECTIONS OF KELLOGG CARBON ARRESTER. 

the spring connecting with the removable head of the coil, thence, 
when the coil is intact, through the winding of the coil and through 
the spring which holds the other end of the coil through which the 
circuit is led, to one of the left hand terminals of the arrester, 
through the insulated tube of brass surrounding the bolt which holds 
the entire structure together. Ordinarily this circuit is entirely in- 
sulated from all other portions of the arrester, but when the heat 
coil is traversed by too great a current, it is pulled apart by springs, 




FIG. 440.— STROMBERG-CARLSON CARBON ARRESTER FOR- 
TELEPHONE SETS. 



thus allowing the spring which connects the line to become grounded 
by virtue of an extension of the line spring which then presses 
against the strip upon which all the arresters are mounted; this 
strip is in all cases well grounded at the central office. The circuit 



PROTECTIVE DEVICES. 



G07 



through the left hand arrester may be traced by following arrow, 
b, from the switch-board wire through the bolt, and thence from 
the outside spring of the left hand heat coil through this coil and 
to the line spring as shown. The two pairs of carbon plates are 
normally held between the ground plate and two springs which are 
permanently in contact with the line springs carrying the heat coils. 
These springs are not, however, normally grounded because of the 



G/SOi/A/O Ptstre 



TO 1./A/Z 




TO SW/TC//3O/7X0. 



FIG. 441.— COOK COMBINED HEAT COIL AND CARBON ARRESTER. 

presence of the mica strips between the carbon blocks. The ground 
plate or strip is usually of German silver or brass, and is supported 
by and in electrical contact with the heavy iron strip upon which all 
of the arresters are mounted. The various springs are guided in 
their respective movements by means of two hard-rubber posts, 
which pass through each of the springs and which also carry con- 
ducting pins for the purpose of completing the alarm circuit when 
the heat coil operates. For this purpose there is an additional 



608 



AMERICAN TELEPHOXE PRACTICE. 



spring on each arrester, adapted, when the heat coil releases the 
springs, to close the alarm circuit and thus indicate to the attendant 
that the line has been opened at the heat coil. Frequently each of 
these alarm circuits is made common to a single strip of arresters 
and an annunciator placed in the circuit, so that by a glance at 
the annunciator-board the attendant may tell on which strip of 
arresters the coil has been operated, thus saving time in effecting its 
repair. 

Sometimes an added feature of protection is provided by inserting 
in one of the carbon blocks of each pair a small drop of solder or 
easily fusible metal. If an arc occurs between the two blocks this 
metal will melt, thus establishing a more or less perfect connection 
between the two and affording a better grounding of the line. 

As will be shown in a subsequent chapter the arresters in central 
offices are usually mounted on the switch-board side of the main 




c 



FIG. M2.-KELLOGG ARRESTER WITH TEST PLUG INSERTED. 



distributing frame, and when so mounted they afford the most con- 
venient place on the line circuit within the office at which to make 
electrical tests as to the condition of the line both outside or within 
the office. 

In the general form of arresters shown in Fig. 441, such tests are 
usually made by removing the heat coils of the pair of arresters 
belonging to the line to be tested, and inserting in their place a four- 
contact plug, so arranged as to make contact with the two-line 
springs and with the two springs connected with the switch-board 
circuits. By means of the conductors leading from this plug, the 
party making the tests has access to both ends of the line for testing 
purposes, that is, to the two line wires leading from the central office 
to the subscriber's premises and to the two wires leading to the 
arresters from the switch-board. 

Fig. 442 shows such a test plug as applied to a pair of arresters; 



PROTECTIVE DEVICES. 



009 



this figure also incidentally showing the type of arrester put on the 
market by the Kellogg Switch-Board and Supply Company. The 
general plan of operation of this arrester is the same as that de- 
scribed in connection with Fig. 441, but the "heat coil" is peculiar 
and merits attention. Instead of using a German silver wire as a 




FIG. 443.— KELLOGG HEAT COIL. 

heat-producing medium, a small cylindrical block or carbon, a, Fig. 
443, is used, and to this is soldered the brass terminals which serve 
to engage the springs of the arrester holder. The size and general 
appearance of this device is not unlike that of the ordinary heat coil 
of the type shown in Fig. 425, and although it is not in any sense a 
coil, it is ordinarily called, by that name. This device depends for 
its operation on the heat generated by the passage of a current 
through the carbon, which is made of suitable resistance to produce 
the desired degree of heating. Before soldering the brass terminals 
on the ends of the carbon block the ends of the latter are copper 





FIG. 444.— AMERICAN ELECTRIC FUSE COMPANY'S SELF-SOLDERING 

HEAT COIL. 



plated and the terminals then soldered in place. The solder used 
in this and other heat coils is usually of such alloy as to melt at 
about 160 F. 

All of the heat coils mentioned are subject to one objection, 
which, however, has been brought to notice only in view of an in- 



610 



AMERICAN TELEPHONE PRACTICE. 



genious improvement recently made. This objection is that when 
once used it was necessary to throw the coil away or to go to some 
expense to put it in proper condition to be used again. 

In Fig. 444 are shown three views of a type of coil embodying 
this improvement. This is manufactured by the American Electric 
Fuse Company, of Chicago, and was designed by Mr. C. A. Rolfe, 
of that company. In this, the movable part, instead of entirely 
separating from the body of the coil when the solder melts, is merely 
moved from one position to another, and in so moving releases the 
spring which grounds the line. As soon as the spring has been so 
released the circuit through the coil is broken and the parts imme- 
diately begin to cool. The movable part as soon as sufficiently 



1 




FIG. 445.— ROLFE ARRESTER. 



cooled is thus automatically resoldered into its new position in 
which it is, by its other end, again adapted to hold the spring as 
before. 

Referring particularly to Fig. 444, the only movable part of this 
device is a small lever shown at the right-hand end of the coil. 
When it is in the position shown in the upper left-hand view of this 
cut, the spring of the arrester holder is adapted to be caught under 
the lower leg of this lever and thus retained in place. When the 
solder holding this lever in this position is softened the spring forces 
the lever into the position shown in the upper right-hand view of 
this cut and the spring is thus released. After the coil is cooled it 



PROTECTIVE DEVICES. 



611 



is only necessary to turn it around in the holder in order to bring 
the other end of the lever in connection with the spring, as that end 
is now in such position as to retain the spring. This device may be 
operated many hundreds of times without showing any appreciable 
deterioration, and as a result the only expense caused by the blow- 
ing of the heat coil is that of the time necessary for the attendant 
to again set the springs which it released by this operation. 

Another type of thermal arrester or heat coil which deserves pass- 
ing mention, is that also designed by Rolfe and shown in Fig. 445. 
In this the heat coil assumes the form shown in the upper portion 
of this figure, it consisting of a little metallic capsule having em- 
bodied within it a coil of German silver wire held in place and in- 
sulated by a plastic material. resembling sealing-wax. A hook -pro- 
jects from the small end of this capsule, thus forming one end of 




FIG. 446.-ROLFE ARRESTER. 



the coil, the capsule itself forming the other terminal. This device 
is held between a coiled spring and a forked clip, as clearly shown 
in Fig. 445, the heat coil being thus subjected to tension and also 
forming a link in the circuit to be protected. When traversed by 
too heavy a current the hook pulls out of the capsule, due to the 
softening of the plastic material, and the circuit is broken by the 
retraction of the coiled spring. 

These arresters are manufactured by the American Electric Fuse 
Company, and so made as to adapt them to service in place of the 
ordinary Western Union fuse, the working parts of the arrester 
being mounted on a fibre strip provided with clips adapted to slide 
into the fuse holder in place of the ordinary fuse. This arrange- 
ment is quite clearly shown in Fig. 446 as applied to the Western 
Union style of fuse holder. 



CHAPTER XXXI. 

DISTRIBUTING FRAMES. 

In every central office it is necessary to provide some means for 
distributing the various line wires which enter the office to their 
proper numbers on the switch-board and switch-board apparatus, 
and to afford means for changing this distribution as required. To 
do this in any manner without a proper regard for systematic ar- 
rangement would lead to endless trouble by producing the tangle 
of wires commonly and well-termed "rat's nest." 

In order to provide means for the systematic arrangement of 
wires and for the proper handling of subsequent changes in their 
distribution, what is called the distributing board or frame is used. 
These two terms are at present used indiscriminately. The boards or 
frames assume a great variety of forms, but the principle on which 
they are designed is as follows : On one portion of the distribut- 
ing frame are placed clips or connectors, suitably arranged, in which 
the wires of the cables leading from the lines may terminate. On 
another portion of the distributing frame is arranged another set 
of clips, in which another set of wires leading to the switch-board 
apparatus may terminate. All conductors from the lines, and all 
conductors from the switch-board apparatus are wired in a per- 
manent manner to the various connectors or clips on the respective 
portions of the distributing frame. The gap between the terminals 
of any pair of wires on the line portion of a distributing frame, and 
those of the corresponding pair leading from the switch-board 
apparatus, is filled by means of bridle or jumper wires, the latter 
term being the most commonly used. The distributing board is so 
arranged that the jumper wires may lead from any pair of con- 
nectors on the line portion to any pair on the switch-board portion, 
and if the distributing frame is a good one these jumper wires 
may be led with perfect order and may be changed as often as 
desired. 

In Fig. 447 is shown a distributing board, which is, happily, a 
relic of the past, but which, in its day, was distinctly superior to . 
others, and may be said to have served its purpose well. It is of 
interest here from a historical standpoint. This picture was taken 

612 




FIG. 417.— OLD ST. LOUIS DISTRIBUTING FRAME, 
613 



614 



AMERICAN TELEPHONE PRACTICE. 



from the inside of the distributing frame, which was in the form 
of a hollow square, one corner of which is shown. It accommodated 
about 4,000 lines entering the old office of the Bell Telephone Com- 
pany, at the corner of Fourth and Pine streets, in St. Louis, Mo. 
The lines were practically all aerial and were brought to a large 
tower at the top of the central office building; they were then led 
down in cables, shown in the picture, these cables being fanned out 
for attachment to the clips on the upper portion of the frame. In 
the same manner the switch-board cables were led up to the lower 
set of clips on the distributing board shown at the bottom of the 





£_]=» 



FIG. 448.— EXD VIEW HIBBARD DISTRIBUTING FRAME. 



picture. Connection between any line wire and any switch-board 
wire was then completed by means of a jumper wire, as readily 
seen. 

A later form of distributing frame which at one time was widely 
used by the Bell companies, was the design of Mr. Angus S. Hib- 
bard, and is illustrated somewhat in detail in the accompanying 
figures. 

The frame is built up entirely of iron pipes, extending in three 
directions and mounted upon a hollow platform, a, shown in Figs. 
448 and 449. These two figures represent respectively the end and 
side elevations of the complete framework, a plan view being shown 



DISTRIBUTING FRAMES. 



615 



in Fig. 450. Vertical pipes serve as supports for the structure, and 
are intersected at short intervals by transverse pipes, i, and lon- 
gitudinal pipes, e, f, and g, extending the entire length of the frame- 



l 

I 

E 
L 



K- 



\ 



1 
1 



H 



^ 



_. 



I 1 y if- — y !,j y y y — [,' y y y y y 1 

1 1 V \ I \ U \ 1 1 1 \ I 



L 
E 



HQ 



:: 



FIG. 449.— SIDE VIEW HIBBARD DISTRIBUTING FRAME. 

work. As a result of this arrangement channels or horizontal runs 
are formed for the jumper wires between the vertical and lateral 
bars, and vertical channels or falls between the sets of intersecting 
horizontal bars. On the ends of the lateral bars, i, are vertical 
strips, d and d' , of insulating material, upon which are arranged 
the terminals for the various wires in the cables and the jumpers. 



1 1 hi 1 1 1 i i 1 1 1 f i 1 i r 



J i I I I I I I i I 1 t i I I i I 

FIG. 450.-PLAN VIEW HIBBARD DISTRIBUTING FRAME. 

The general plan by which the wires in old magneto exchanges 
were led from the cable heads to the switch-board is shown quite 
clearly in Fig. 448, where I! represents the cable head carrying 



616 



AMERICAN TELEPHONE PRACTICE. 



the terminals of the line cable, C. The various wires, w, leading 
from the cable head are bunched j^ito a cable, C 2 , which enters the 
cable run in the box beneath the frame, and after passing in a 
horizontal direction to the proper insulating strip, d' , is led upward 



€ 



m 

u u 



Of 



tt 




u u 
FIG. 451.— DETAIL OF CONNECTING STRIPS. 



and fanned out, the various pairs of wires being soldered to the 
outer ends of the terminals on the insulating strip. 

The details of these strips and the methods of attaching the 
wires of the cable are shown in Fig. 451, in which p and p' are 
the connectors screwed to the strip, d. These connectors have 
outwardly bent lugs, u, to which the wires may be soldered. 




FIG. 452.— ENLARGED PLAN HIBBARD FRAME. 



The ends of the jumper wires are shown at t f. In a similar manner 
the wires leading from the switch-board jack are bunched into a 
cable, C 3 , which is then led through the cable run to the proper 
strip, d, of the distributing board, where it is fanned out and con- 



DISTRIBUTING FRAMES. 



617 



nected to similar terminals. The vertical portions of the cables, 
which are to be fanned out on the distributing board, are sup- 
ported by the lateral horizontal rods, i, by being laced thereto, this 
being shown quite clearly in the enlarged plan view of Fig. 452. 




FIG. 453.— FORD & LENFEST DISTRIBUTING FRAME. 



The jumper wires, which are usually of tinned rubber-covered wire 
in twisted pairs, are attached to the inner ends oi the terminals 
on the line side of the distributing board and led through a hole in 
the strip and through the proper channels in the framework to the 



618 



AMERICAN TELEPHONE PRACTICE. 



desired terminals on the switch-board side, where they are secured 
in the same manner. 

This arrangement serves to keep the wires fairly open and easy of 
access. It has several great distadvantages, however, the principal 
one of which is that it is too cumbersome. It has been almost en- 




FIG. 454.-DETAILS OF FORD & LENFEST DISTRIBUTING FRAME. 



tirely superseded in the practice of the Bell companies by a frame 
designed by Messrs. Ford & Lendfest, some of the details of which 
are shown in Figs. 453 and 454. This, like the Hibbard frame, 
is in the form of an open framework built almost entirely of iron 
so as to be practically fireproof. Iron bars, 1, 2 and 3, to which 



DISTRIBUTING FRAMES. 619 

are bolted plates, 4, form the foundation of the frame. To the face 
of the plates, 4, are bolted the supporting columns, D', of angle iron, 
to which are secured all the other portions of the frame. Horizon- 
tal bars, 6, are bolted to the columns, D', and carry upon one side 
of the frame horizontal strips, 8, of hard wood, upon which are 
secured the terminals for the wires of the, street cables. A detail 
of these terminals is shown in Fig. 454, the metallic connectors, m 
and n, being secured in place in transverse saw-cuts in a thin strip 
of hard rubber by another strip bolted over them. Supported upon 
the other end of the horizontal bars, 6, are the vertical pieces, 10 and 
11, of hard wood and the flat bar, 12, which is of iron. Upon this bar 
of iron are mounted the arresters, X and Y, as shown in Fig. 454. 
These arresters, which are of the combined static and sneak-current 
type, will be recognized as similar to those shown in Fig. 441. 

In wiring this distributing frame, the street cables, C, are led in 
a horizontal direction under the strips, 6, as shown in Figs. 453 and 
454. These cables are then fanned out, the various pairs of wires 
passing through holes, 14, in the under side of the horizontal 
wooden strip, 8, and secured to the lower ends of the connectors, m 
and n. The switch-board cables, L, shown in Figs. 453 and 454, 
are led from beneath up along the sides of the bars, 6, between the 
supporting bars, D' , and the wooden strips, 10 and 11. They are 
supported in this position by being laced to the horizontal bars 
themselves. These cables are fanned out, the various pairs passing 
through holes, 15 and 16, in the wooden strips, 10 and n, and to 
their appropriate terminals on the arresters. The connections of 
the street and switch-board cables are thus as far as possible made 
permanent. The jumper wires are each led through a hole, 13, in 
the upper part of the horizontal wooden strip, 8, its ends being 
secured to the upper portion of the connectors, m and n, as shown 
in Fig. 454. The pair is then led in a horizontal direction along the 
top of the bars, 6, on the line side of the frame until a point is 
reached opposite the vertical strip on which the desired switch- 
board terminal is located. It is then led through an eye or ring, r, 
and through holes, 17, in the vertical strip, 11, and attached to the 
proper pair of terminals on the arrester through which the connec- 
tion is made with the switch-board wires. 

A distributing frame built upon this general plan is shown in 
Figs. 455 and 456, these being views of the frame in the main ex- 
change of the Bell Company at St. Louis, Mo. 

The line cables approach the frame under a false floor, ami are 




620 



DISTRIBUTING FRAMES. 621 

accessible through trap-doors, as may be seen in Fig. 455. They 
are fanned out on the horizontal side of the distributing frame, as 
shown in this figure. The terminals on the line side are numbered 
with respect to the wires in the cables to which they belong. On 
the vertical side of this board, which is shown in Fig. 456, are placed 
the arresters, to which lead the wires from the switch-board cables. 
The jumper wires connecting the horizontal with the vertical sides 
are arranged as already described. 

In the Independent field a distributing frame has recently come 
into extensive use similar to the Ford & Lenfest frame, but 
differing from it in that the line side terminals are mounted in 
vertical instead of horizontal rows, and are divided into short 
lengths, thus affording arm space between them for reaching into the 
horizontal jumper runs. A single section of this frame with its line 
strip divided into five short strips of twenty terminals each is shown 
at the left in Fig. 457. This frame is manufactured in large quan- 
tities by Frank B. Cook, of Chicago, for the Stromberg-Carlson 
and other manufacturing companies. A complete distributing frame 
built on this plan with four sets of uprights is shown at the right 
in Fig. 457. 

The distributing frame largely used by the Kellogg Switch-Board 
and Supply Company differs from any of those yet considered in that 
the line terminals and the switch-board terminals are mounted on 
alternate vertical strips arranged on the same side of the frame in- 
stead of on opposite sides, as on all other modern frames. Thus, 
when the arresters are mounted in conjunction with the switch- 
board terminals, as is usually done, there are alternate vertical rows 
of arresters and of line terminals. The jumper wires lead from 
their respective line terminals through iron rings such as those used 
in the Ford & Lenfest and Cook boards, to the opposite side 
of the frame, and then pass through horizontal runways similar to 
those shown in Fig. 453, to a point opposite the proper vertical 
strip of arresters, and then through another ring back to the ter- 
minal side of the frame. The construction is similar to that of the 
Ford & Lenfest and of the Cook frames. 

Up to this point only those distributing frames have been con- 
sidered by which changes may be made with respect to the connec- 
tion between outside lines and lines leading to the switch-board 
apparatus. Such frames are termed main distributing frames, to 
distinguish them from intermediate distributing frames, which have 
an entirely different function 



nj%Mm^mmhMLM^mmm 



/ W% XW TJJTBI. 

■ f / * / * .... n « 

. .-.. . 2 & ...v. 





IStiW^ 



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■ m iiiminu. 





622 



DISTRIBUTING FRAMES. 



623 



In modern exchanges of sufficient size to employ multiple switch- 
boards an intermediate distributing frame should be provided, and 
the circuits leading from the switch-board side of the main frame 
should lead through this before passing to the switch-board proper. 

The function of the intermediate distributing frame is this: It 
affords means for changing the relation between the answering jack 
with lamp and the multiple jacks on any line. Unless a change is 





FIG. 457.— COOK DISTRIBUTING FRAME. 



made at the main distributing frame, a certain line entering the ex- 
change is always definitely associated with a certain number in the 
multiple jacks. Furthermore, the number of a subscriber's line 
must of necessity always correspond with the number of the mul- 
tiple jacks to which it is connected. When a change is made at 
the main distributing frame affecting the relation o\ an outside to 
an inside line, the number of an outside line must oi necessity 
change to correspond with that of the multiple jacks ^i the inside 



624 AMERICAN TELEPHONE PRACTICE. 

line to which it is connected. This is not necessarily true, how- 
ever, with respect to the answering jacks. A little thought will 
show that it is never necessary for an operator to know the number 
of an answering jack, or for her answering jacks to be in numerical 
order, for the only time that she plugs into such a jack is in response 
to the lighting of the line signal immediately below it. For this 
reason, therefore, it is not essential that the answering jack and its 
lamp shall bear a permanent relation to the multiple jacks, and in 
fact, the reverse is quite true. 

The load of any subscribers' operator is determined by the number 
of answering jacks and lamps placed before her and on the amount 
of traffic on the lines served by those answering jacks. On an 
average, in modern common battery exchanges, perhaps 120 lines 
may be handled by an operator; but on account of the great differ- 
ence between the traffic on different classes of lines it may be neces- 
sary to reduce the number of answering jacks on some operator's 
position to one-third or one-quarter of this, while on the other hand, 
other operators, no more skillful, may be able to handle the answer- 
ing jacks of 300 or 400 lines. As just pointed out, it makes no 
difference on what position an answering jack is located so long as 
it has associated with it the proper line lamp and so long as the 
number on the multiple jacks corresponds to that of the line served. 
By means of the intermediate distributing board an answering jack 
and lamp on any position of the board may be made to serve any 
line and its multiple jacks; so that if a certain operator is over- 
loaded, some of her lines may be disconnected from her answering 
jacks and lamps and connected with those of another operator at 
a new position or at some old position at which the operator is not 
overworked. 

The method of making changes at the intermediate frame varies 
somewhat by reason of the requirements of the particular line cir- 
cuit used. In no system, however, should the relation of the mul- 
tiple jacks to the main line be changed at the intermediate distrib- 
uting board, while in all systems changes at the intermediate board 
should alter the relation of the answering jack and lamp with re- 
spect to the multiples and the outside line. In some systems the 
arrangement works out better if the line and cut-off relays occupy 
a permanent relation to the outside line and the multiples, and are 
therefore not subject to change at the intermediate boards. In 
other cases each line and cut-off relay occupies a permanent rela- 
tion with respect to the answering jack and lamp, and therefore 



DISTRIBUTING FRAMES. 



625 



their relation with the line is changed whenever a change is made 
at the intermediate board. To express this in a different way: In 
some systems, as for instance, that of the Kellogg Company, a line 
and cut-off relay always stay with a particular number in the mul- 
tiple jacks regardless of changes at the intermediate frame, and 
therefore the line and cut-off relays always bear the same number 
as the multiple to which they are connected, and therefore the same 
number as the line. In the Western Electric system, however, a 
given line and cut-off relay always stay connected to a particular 
answering jack and lamp, and therefore these relays change with 
relation to the multiple jacks and the outside line whenever a 



•Cr — O 

MAIN 



s 



rnortcTons 



JJOA.B 



*6 



JUMPERS 



*r 



"^z 



IMTCRMEDIA.TE 
D13T BOAK.P 




£=33 



c c 

II 1 V i i i y f ii if 1 
MULTIPLE JACKS 




FIG. 



458.— DIAGRAM OF CONNECTIONS THROUGH INTERMEDIATE 
DISTRIBUTING FRAME— WESTERN ELECTRIC SYSTEM. 



change is made at the intermediate board. The line and cut-off 
relays are therefore given numbers of their own, and do not bear 
the same numbers as the multiple jacks and subscribers' lines which 
they serve. 

To make this clearer, reference is made to Fig. 458, which shows 
the part that the intermediate frame plays with respect to the line 
circuit of the Western Electric multiple switch-board system. 

The line will be seen entering at the left and passing through the 
two sides of the main distributing frame, the jumper wires on this 
frame being indicated in dotted lines. From the main distributing 
frame the two sides of the line are continued to two clips on the 
intermediate distributing frame to which same clips arc run. the 

40 



626 



AMERICAN TELEPHONE PRACTICE. 



tip and sleeve sides of the multiple jacks, the test wires on the 
multiple jacks being run to a third clip on the same side of this 
frame. All of the connections to the line and cut-off relays, also 
to the answering jack and lamps are made to four clips on the 
opposite side of the intermediate distributing frame, all of these 
connections being permanent, therefore allowing of no changes in 
the relation between the answering jack and the line and cut-off 
relays. By means of a three-strand jumper wire, shown in dotted 
lines, it will be seen that the connection between any outside line 
and its multiple jacks and any group comprising, an answering jack, 




FIG. 459.— DIAGRAM OF CONNECTIONS THROUGH INTERMEDIATE 
DISTRIBUTING FRAME— KELLOGG SYSTEM. 



lamp and relays, may be altered by changing the connection of the 
intermediate wires. 

In Fig. 459 is shown the method of accomplishing the intermediate 
distributing board changes when applied to the two-wire systems 
of the Kellogg Company. 

In this it will be seen that the line and cut-off relays are both 
permanently wired to the multiple jacks and to the arrester side 
of the main distributing frame. The tip and sleeve side of the 
circuit leading from the cut-off relay and also from the multiple 
jacks terminate in two clips on the line side of the intermediate 
distributing frame, while the lamp circuit, extending from one of 
the contact springs of the line relays, terminates in the third con- 
tact on the same side of the intermediate frame. No change in 



DISTRIBUTING FRAMES. 627 

the relation between the multiple jacks, the arrester, the line and 
cut-off relay is therefore possible with this form of circuit, all of the 
wiring between them being permanently done. The opposite side 
of the distributing frame has three clips for each line, two of these 
leading to the tip and sleeve strands of the answering jack, the 
other leading to the line lamp, the opposite terminal of which is 
grounded. The answering jack and line lamp may, therefore, be 
made to occupy operative relation with any number in the multiple 
and, therefore, with any outside line, by changing the three-strand 
jumper wires on the intermediate frame. 

It is thought that the latter method, by which the answering jack 
and line lamp alone are affected by changes at the intermediate frame 
is the better. It has the advantage of allowing the numbers of the 
line and cut-off relays to correspond with those of the multiple jacks 
which are permanent. This seems to offer a practical advantage 
in the facility with which cases of trouble may be handled. Again, 
when answering jacks and lamps are furnished in excess of the 
number of lines in the multiple, as is frequently done to afford lati- 
tude in the use of the intermediate frame, the addition of a corre- 
sponding number of line and cut-off relays is not required with 
the former method, it being only necessary to provide one of each 
for each line in the multiples. 



CHAPTER XXXII. 
CHIEF OPERATOR'S AND MONITOR'S EQUIPMENTS. 

The apparatus and circuits by means of which such employees 
as the chief operator, wire chief, monitor and service clerks are en- 
abled to perform their various duties are usually mounted in the 
desks at which these employees sit. An intelligent arrangement of 
these desk equipments with respect to the needs of a central office 
force, will do much toward economy and good service in general. 

Practice concerning desk equipments varies widely among the 
different operating companies, but in recent years that of the more 
progressive Bell companies and some of the larger Independent 
companies seems to have settled into channels well enough defined 
to be called standard. 

The principal employees, directly or indirectly concerned, in the 
giving of service, and who rank above the regular operators, are 
the chief operator, monitors, supervisors, and wire chief. All of these 
except the supervisor are, as a rule, provided with desk equipments. 
The duty of the supervisors, in most exchanges, is to walk up and 
down behind the regular operators, watching the service and help- 
ing out where occasion require." Since the nature of their duties 
keeps them on their feet, they require no desks. 

It is perhaps the more common practice to place the chief opera- 
tor's and monitor's equipment in the same desk, this being large 
enough to allow both the chief operator and monitor to occupy it 
at once. During the least busy time either alone may attend to the 
duties of both. 

The chief operator, being in charge of the entire operating force, 
is usually given the following facilities for directing, supervising 
and inspecting the work of this force, as well as for giving personal 
attention to those subscribers who may require it : 

For observing the service given on any subscriber's line about 
which there has been complaint, the chief operator is usually pro- 
vided with a number of lines terminating in jacks and lamps on her 
desk, and at normally open clips on the intermediate distributing 
frame. When persistent or serious complaint is received from a 
subscriber as to the service he is obtaining, one of these lines for 

628 



CHIEF OPERATOR'S AND MONITOR'S EQUIPMENTS. 629 

observing service may be connected at the intermediate distributing 
frame with that subscriber's line, and left so connected for a period 
sufficient to allow the chief operator to make a thorough diagnosis 
of the case. When a subscriber's line is so connected, a lamp will be 
lighted on the chief's desk, simultaneously with the lighting of the 
line lamp on the regular switch-board in response to the call of the 
subscriber. The chief operator, who is provided also with a tele- 
phone set terminating in a plug, may, in response to this signal, plug 
in on the line and' thus observe the response of the A operator, 
noting the time and the method of the response. The chief may also, 
during the conversation, observe the language and demeanor of the 
subscriber in asking for a connection, and determine any irregulari- 
ties which may exist at either end of the line. In connection with 
each one of these lines for observing service is another lamp on the 
chief's desk which lights whenever the subscriber on a line that is 
under surveillance is called by an A or B operator. The chief oper- 
ator noticing the lamp, may go in on the line with a plug for the 
same purpose as before. These lines, therefore, which are called 
"service observation" lines, afford means for the chief operator to 
personally investigate the justice of complaints as to service by sur- 
reptitiously watching the behavior of the subscriber and of his 
operator in answering his call, and of any operator in calling him up. 

The chief operator is also provided with several lines terminating 
in. her desk in jacks and lamps, and multipled through the main 
switch-board as regular subscribers' lines. (One or more strips of 
jacks at each section, are usually set aside in the multiple in the 
main board for accommodating lines leading to the various desks.) 
By means of these lines leading to he chief operator's desk, the chief 
operator may be called by an A or B operator in the same manner 
as a subscriber would be called, and if need be, the chief operator 
be put in communication with any subscriber over these lines. In 
this way the A or B operators are enabled to refer any subscriber 
whose business demands it, directly to the chief operator, thus re- 
lieving these operators from all duties which might detract from 
their regular work. 

When the chief operator and monitor occupy the same desk, this 
is usually equipped with lines called "monitor's taps." There is 
one of these for each regular operator in the office, each leading 
from a jack on the desk to the head telephone of an operator. By 
inserting a plug into any one of these jacks the chief operator or 
monitor is enabled to listen to the conversation of a regular operator 



630 



AMERICAN TELEPHONE PRACTICE. 



without her knowledge. When the chief operator's or monitor's 
desks are separate, these taps terminate in the monitor's desk. 

In addition to the different circuits mentioned, the chief operator's 
desk is usually provided with lines leading to various local points 
in the exchange, such, for instance, as a line to each of the other 
desks, a line to the traffic manager's office, and lines to the office 
of any other official who may have need for direct communication 
with the chief operator. 

It is the custom of many of the independent companies, but as 
a rule not of the Bell companies, to provide on the chief operator's 
or monitor's desks, a lamp for each position on the main switch- 
board, this lamp being wired so that when the calling pilot lamp at 
any position of the switch-board lights, a corresponding lamp on 
the desk will light. In those systems where the supervisory signals 
at the main board are also provided with pilots, this feature is 




FIG. 460.-CHIEF OPERATOR'S TELEPHONE CIRCUIT. 



usually extended also to the chief operator's and monitor's desk, 
the arrangement being such that two lamps are provided for each 
position on the main board, one adapted to light whenever the line 
pilot at that position lights, and the other whenever the supervisory 
pilot at that position lights. For convenience in distinguishing 
between the monitor's lamps, those corresponding to the line pilot 
are made white, and those corresponding to the supervisory pilot 
are made red. By this means the chief operator or monitor, seated 
at the desk, may, by noting the length of time that any one of the 
monitor's lamps remains lighted, determine the promptness with 
which calls are being handled on various positions of the board. If 
it is seen that a certain operator's work is dragging, she may be com- 
municated with over one of the circuits leading to that position, and 
if found in need of help it may be provided. 



CHIEF OPERATOR'S AND MONITOR'S EQUIPMENTS. 



631 



An almost endless variety of desk equipment circuits might be 
given, these, of course, differing for each general exchange system r 
and also with the ideas of the various operating companies. M odern 
Bell practice may be taken as typical, and therefore all of the dia- 
grams in this chapter will, unless otherwise specified, illustrate the 
latest obtainable ideas of the Bell companies on this subject. These 
circuits are all adapted to use in connection with the standard com- 
mon battery system supplied by the Western Electric Company. 

Fig. 460 is the chief operator's telephone circuit, adapted to con- 
nect by means of its plug with any line terminating at the chief's 
desk. A retardation coil bridged across the cord circuit is for the 
purpose of affording a comparatively low resistance path for direct 



/v 



s 



/* 



I 



mm 



■^^j 



Oa 



OOO 

INTER. DIST. FRAME 

FIG. 461.— SERVICE OBSERVATION LINE. 



current, so that when the chief operator plugs in on a line termin- 
ating as a subscriber's line in the main switch-board, the effect will 
be to light the lamp of that line in the same manner as if the sub- 
scriber had removed his receiver from its hook. This retardation 
coil may be cut out by means of a key, as shown. Besides the 
regular listening key by which the chief operator may associate her 
talking set with any one of the plugs with which she is provided. 
there is another key which, when operated, serves to cut out 
the chief operator's transmitter only. The object of this is to pre- 
vent the subscribers' operators from knowing when the chief oper- 
ator listens in on their circuits. Operators become very skilled in 
noticing any changes in the sound of their circuits, and would at once 



632 



AMERICAN TELEPHONE PRACTICE. 



observe the effect of any other transmitter being connected with 
their telephone sets. 

As illustrating the astute methods sometimes employed by the 
regular operators to ascertain whether or not they are being watched, 
it may be mentioned that in some cases where nickel-plated fronts 
have been provided for the operators' transmitters, these have been 
kept carefully polished and used as a mirror, by which the operator 
could observe the actions of the chief, seated behind her. This is 
one reason for the usual black finish of the operators' transmitters. 

In Fig. 461 is shown one of the chief operator's service observa- 
tion lines. These lines are normally terminated at open clips on the 



11 



MULTIPLE JACKS 



< 



TEMPORABY JUMPERS 



■__£ 



wTL_.T.__ ( fL™. 



m 



INTER. DI3T. FRAME 




25llft 



fiSl 



CHIEF OFTV5. DESK.. 



irse 



ANS.OA.CK 



MULTIPLE PO\RD. 



FIG. 462.— METHOD OF CONNECTING LINE FOR OBSERVING SERVICE. 



intermediate distributing frame, and by means of three-wire jumpers, 
any one of them may be connected to any subscriber's line. A 
regular Western Electric line circuit so connected by means of a 
temporary jumper wire to a service observation circuit on the chief 
operator's desk is shown in Fig. 462. 

When the subscriber on such a line calls the lamp, A, will be 
lighted, that lamp being connected between the live side of battery 
and the regular line lamp terminal on the intermediate frame, the 
two lamps thus being lighted in multiple by the action of the line 
relay. The chief operator, by inserting the plug of one of the cir- 
cuits shown in Fig. 460, will place her telephone in a bridge of 



CHIEF OPERATOR'S AND MONITOR'S EQUIPMENTS. 



633 



the subscriber's line, cutting off the remaining portions of the 
observation circuit apparatus at the cut-off jack, but not putting 
out the lamp, A. This lamp will go out when the A operator 
answers the call. When a regular operator rings out on such sub- 
scriber's line, the restoring drop, D, at the chief's desk is thrown 
through the condenser, C, thus allowing the lamp, B, to light. The 
drop is restored in the usual manner by the insertion of the chief 
operator's plug, thus putting out the lamp. The circuit over which 
this drop is restored includes the third strand of the chief operator's 
plug and the sleeve contact of the jack. 

The circuit by which the chief operator may be brought into 
connection with any subscriber through the main board, or by which 
any operator may put herself into communication with the chief 



■ec 




CHIE.F OPERATOR 



«JJ 



^^^_J 



FIG. 463.— CHIEF OPERATOR'S LINE TO MULTIPLE BOARD. 



operator, is shown in Fig. 463. The apparatus of the chief operator's 
board is shown at the right-hand side of this figure, the apparatus to 
the left-hand side being that of any regular common battery sub- 
scriber's line. The chief operator, by inserting one of her plugs 
(Fig. 460), into the jack of such a line on her board, may, by 
means of the retardation coil in her cord circuit, call the A operator 
at the position where the line lamp of this line is placed. On the 
other hand, any regular operator, by going in one of the multiple 
jacks and ringing, will throw the self-restoring drop on the chief 
operator's desk, lighting the lamp belonging to this line. This 
lamp will be put out by the restoring of the drop over the third strand 
of the cord of the chief operator's plug. 



634 



AMERICAN TELEPHONE PRACTICE. 



The monitor's taps from the chief operator's desk to the various 
positions of the regular board shown in Fig. 464, need no explana- 
tion in view of what has already been said. 



(Ik 



■a ,o- 



OPR3.TCL.. 
SET. 



FIG. 464.— MONITOR'S TAP. 



In Fig. 465 is shown the circuits of one of the lines leading from 
the chief operator's desk to the various local points, as, for instance, 
to the local telephone set of the traffic manager. At the right of 
this figure is shown the line leading to the local telephone equip- 



jfX 




FIG. 465.— CHIEF OPERATORS' LINES TO LOCAL POINTS. 



ment. At the chief's desk this passes first through a ringing key, 
K, thence normally through retardation coils and a relay, R, to 
the common battery. When the user of the local telephone con- 
nected to this circuit removes his receiver from its hook, the lamp, 
L, associated with this line on the chief operator's desk will light. 
It will be put out when the operator responds by throwing the 
key, K'j which will connect her telephone set with the line calling. 
The two may thus converse. When, on the other hand, the chief 
operator wishes to call up a party on one of these local lines, she 
has only to throw the corresponding ringing key, K, to ring the 
bell of such subscriber, after which the operation is as before. 



CHAPTER XXXIII. 

WIRE CHIEF'S EQUIPMENT. 

The wire chief, by the aid of his desk equipment, exercises one 
of the most important functions in an exchange, his duties relating 
largely to the determination of the electrical conditions of the vari- 
ous outside and inside lines, this information forming the basis for 
the proper repair and maintenance of the lines and line equipments. 
The wire chief's desk may be equipped with one or more positions, 
according to the size of the office and the number of persons likely 
to be required in the performance of the wire chief's duties. 

Extending from the wire chief's desk are, as a rule, a number of 
testing trunks leading to a certain position on the main switch- 
board, where they terminate in plugs. It is customary in order not 
to thus take up room on a position that is available for operating, 
to terminate these testing trunks on one of the end positions of the 
local multiple board, from which position by the use of long cords 
an entire section may be reached by these plugs. Any testing plug 
may thus be inserted into the multiple jack of any subscriber's line. 
Besides these testing trunks leading to the multiple board there are 
other testing trunks leading from the wire chief's desk to the main 
distributing frame, where each terminates in a plug adapted to be 
connected at the arrester with any subscriber's line. By means of 
these testing trunks any subscriber's line may be continued to the 
wire chief's desk for the purpose of testing. 

Lines are also usually extended from the wire chief's desk to one 
or several jacks on each section of the main board, these being mul- 
tipled through the sections in the same manner as the jacks of the 
chief operator's lines. The wire chief's desk is also equipped with 
suitable testing apparatus by means of which the complete electrical 
condition of any subscriber's line may be ascertained in the most 
convenient manner. 

The methods of performing the various tests on the wire chief's 
desk have changed radically during recent years. The old practice 
made use almost entirely of Wheatstone bridge methods, while more 
recent practice uses almost exclusively various voltmeter methods.* 



♦Those not familiar with bridge and voltmeter methods of testing are referred 
Chapter XLI1. on Testing, at the end of this hook. 

G35 




^6] 




WIRE CHIEFS EQUIPMENT. 637 

Although the Wheatstone bridge method is not in accordance 
with what is now considered the most advanced practice for wire 
chief's use, it will be described briefly, as it is in use in a large num- 
ber of exchanges. 

In the right-hand portion of Fig. 466 is shown a testing trunk 
leading from the main distributing frame to the wire chief's desk. 
In the left-hand portion is shown the circuit of a subscriber's line 
leading from the subscriber's station to the switch-board apparatus 
at the central office through the arrester clips at the main board, 
the heat coils having been removed. 

The removal of the heat coils and insertion of the test plug divides 
the line at the distributing frame into two portions, one extending 
out from the arrester to the sub-station and the other extending in 
from the arrester to the switch-board apparatus. The two portions 
so divided may be called the "outside" and the "inside" lines re- 
spectively. The four limbs (two each) of the two divisions of the 
line are led to the wire chief's desk by means of the testing trunk, 
the plug in which this trunk terminates being so arranged as to 
make connection with the proper arrester springs connected with 
the outside and inside lines. 

By means of the jacks, a, b, c and d, on the wire chief's desk, 
plugs in which the testing instruments terminate may be inserted 
and a test made on tip or ring side of either the outside or the inside 
line. When no plug is inserted in any of the jacks and when keys, K 
and K', are in their normal positions as shown in Fig. 466, the sub- 
scriber's station is connected through to the switch-board with the 
addition of the ringer and condenser within the wire chief's desk 
bridged across the circuit. By throwing the key, K', to operate its 
right-hand springs, the wire chief may place his telephone equipment 
upon the circuit of the line. When testing the outer line by use of 
jack b or a, or both, the ringer is left upon the inside line where it 
will signal the wire chief in case the line under test be called in the 
regular way by any A or B operator. When testing the inner line, 
the key, K, is thrown so that a subscriber attempting to call upon the 
line under test would have his call received by the lamp, L, upon the 
wire chief's desk. With the key, K', thrown to operate its left-hand 
springs, ringing signals from the switch-board are kept off of the 
line under test, and are received at the wire chief's desk instead. 

In Fig. 467 is shown the test plug circuit. By means of these 
plugs and the jacks, a, b, c or d, the Wheatstone bridge may be con- 
nected between any two of the four limbs of the inside and outside 



638 



AMERICAX TELEPHONE PRACTICE. 



line, or, by inserting one of the plugs in the ground jack, g, between 
any one of the limbs and ground. 

As will be seen the two testing plugs in Fig. 8 are normally con- 
nected in series through a battery of five cells' and through a relay 
and key, the purpose of which is to ascertain whether or not the 
relay will be operated when the key, K", is closed over whatever 
resistance is connected between the circuit of the two plugs. 

When, however, the key, K"', is thrown into its alternate posi- 
tion, whatever resistance is between the two plugs represents the 
unknown resistance on the Wheatstone bridge arrangement, and 
may be measured accordingly. The voltage of the battery supplied 
to the Wheatstone bridge may be varied by inserting the plug, p, 
into one of the several jacks connected with the cells of the battery. 



OO 




WHEATSTONE BRIDGE. 

FIG. 467. -TESTING PLUG CIRCUIT. 



It is evident that if it is wished to measure the resistance between 
the ring side of the outside line and ground, one of the bridge plugs 
will be inserted in the jack, c, while the other will be inserted in the 
ground jack, g. 

The more modern wire chief's desk equipment used by many of 
the Bell companies are shown in Figs. 468, 469, 470, 471 and 472. 
In Fig. 468 is shown the test trunk leading from the wire chief's 
desk to the test plug adapted to connect with the arrester springs 
at the main distributing frame. Remembering that the two outside 
springs of the arrester lead to the switch-board end of the line, while 
the two inside terminals lead to the outside line, it will be seen that 
when the connection is thus made with a line, the jack, /, affords 
means for connecting directly with the switch-board line circuit, 



WIRE CHIEF'S EQUIPMENT. 



639 



while the jack, /', affords means for connecting with the outside 
line circuit. 

For the purpose of avoiding, as far as possible, any interruptions 
in service while connection is made with a subscriber's line, the 







! ,.■-.£ 



FIG. 468.— WIRE CHIEF'S TESTING TRUNK. 

drops, D and D', are provided, which are under control of the inside 
and outside lines respectively. Thus, while such a connection is up 
at the main distributing board, if the operator at the switch-board 
sends calling current out on the line, the drop, D, will fall, the ring- 
ing current passing through the coil of this drop and the condenser. 




FIG. 469.-WIRE CHIEF'S THROUGH CONNECTING CORDS. 

If, on the other hand, a subscriber attempted to call the exchange 
the drop, D', would fall. In cither ease the wire chief could answer 
by plugging into the jack, / or /', with the plug, /\ of the "through 
connecting cord circuit" shown in Fig. 460, and throwing his listen- 



640 



AMERICAN TELEPHONE PRACTICE. 



ing key. If he so desired and the condition of his line permitted, 
he could then complete the connection by inserting the correspond- 
ing plug, p', in the other jack, either / or /'. As soon as the conver- 
sation was finished, which fact he could ascertain by listening in, 
he would pull down the plugs and resume testing. 

As the jacks, / and /', are of the cut-off type, plugging into either 
of them cuts off the drops and all other apparatus from the line, 
leaving a straight connection to either the outside or the inside line. 
The shutters of the drops are restored in the usual manner upon 
plugging into the jacks, through the third strand of the cord. 

One of the testing trunks leading to the multiple board is shown 
in Fig. '470. This terminates on the wire chief's desk in a drop, 



Mt/L.T; Sw .g p 



WIRE CHIEF'S DE.OK. 



INTE.RRUPTE.R 



_M 



■rat 




wp^ 



TRANSFORMER 



FIG. 470.— TESTING TRUNK TO MULTIPLE BOARD. 



D", and jack, J", the arrangement being exactly the same as that 
of the drop, D, and the jack, J', in the testing trunk to the main 
distributing frame, the circuit of which is shown in Fig. 468. At one 
position of the multiple board these trunks terminate in plugs, 
P, so that any line may be extended to the wire chief's desk by 
inserting this plug in its multiple jack. The circuit of the third 
strand of this plug passes through a lamp, L, and key, K, located 
at the wire chief's desk, and thence through the secondary of a tone 
test transformer to the live side of battery. By means of inter- 
ruptions in the primary of the transformer a tone is put on the 
test rings of a line thus plugged up for test. The cut-off relay of 
such a line is operated in the usual manner, thus rendering the line 



WIRE CHIEF'S EQUIPMENT. 



641 



signal inoperative. By pressing the key, K, however, the wire 
chief may at any time release the cut-off relay of a line thus held. 
By means of a plug and cord circuit, shown in Fig. 471, he may 
then establish connection between the tip and ring sides of the 
test trunk and thus flash the line lamp to attract the attention of 
the A operator. After this he may converse, with the operator by the 
talking set of Fig. 471. 

In Fig. 472 is shown the test circuit by means of which the wire 
chief is enabled to perform all the necessary tests to ascertain con- 
ditions of any line. The circuit is equipped with the following 
apparatus : 

A ringing key, 1, to enable the wire chief to ring out over any 
testing trunk. 

A reversing key, 2, to be used for transposing the two sides of 




FIG. 471.— WIRE CHIEF'S TELEPHONE CIRCUIT. 



a line undergoing test with respect to the testing apparatus, as will 
be described. 

A grounding key, 3, to ground one side of the line under test. By 
means of this and the reversing key the two sides of the line may 
be alternately grounded. 

A volt-meter key, 4, to be operated in connection with : First, a 
battery cut-off key, 8, which disconnects the testing battery from the 
volt-meter, and at the same time connects the high-reading coil of 
the latter to the volt-meter key for use in measuring external volt- 
ages ; second, a battery-switching key, 9, which, in its normal posi- 
tion, connects the 40-volt testing battery to the high-reading eoil of 
the volt-meter, and when thrown, connects the 4-volt battery to the 

41 



642 AMERICAN TELEPHONE PRACTICE. 

low-reading coil; third, a volt-meter shunt key, 10, arranged to re- 
duce the effective low-resistance winding of the volt-meter from 
10,000 ohms to 1,000 ohms, for use in measuring comparatively 
small resistances. 

A relay key, 5, used to connect a relay and sounder to a line 
under test, when audible signals are desired, or when a test with 
a current of greater volume than that allowed by the volt-meter is 
desired. 

A continuous ringing key, 6, to switch the calling generator on 
the line and hold it there while continuity tests through condensers 
or repeating coils are being made. 

A spare key, 7, for the connection of any special apparatus de- 
sired. This is often used for the attachment of a "howler" current 
designed for calling subscribers in case of failure to replace the 
receiver upon the hook. 

A listening key, 11, to be operated in connection with a retardation 
coil key, 12, which normally bridges a retardation coil, R 2 , across 
the talking set, and when thrown disconnects it ; the retardation coil 
is used as a means to cause the subscriber's operator's supervisory 
signal to be extinguished when the wire chief or inspector responds 
to a call from the multiple switch-board. It is also used to attract 
the attention of an A operator over any line or test trunk running to 
the main switch-board. The listening key is also to be used in 
connection with a battery key, 13, which, when thrown, supplies 
battery current to the two sides of the cord circuit ; this combination 
is provided in order to enable a subscriber who has been called up 
for test to converse with the wire chief. 

The various tests commonly employed by the wire chief in locat- 
ing trouble on common battery lines are divided by the Bell Com- 
pany in its specifications for testing with its standard wire chief's 
equipment as follows : First, tests for grounds ; second, tests for 
the continuity of currents ; third, tests for crosses ; fourth, ballistic 
tests ; fifth, tests for continuity through condensers and repeating 
coils ; sixth, tests for determining the presence and magnitude of 
foreign currents ; seventh, tests for locating trouble in the switch- 
board ; eighth, miscellaneous tests. These will be briefly taken up 
in their order. 

Tests for grounds may be made in two ways, by means of the 
volt-meter, or by the use of relay, R, connected through key, No. 5. 
In order to make clearer the methods of volt-meter testing for 
grounds, and of measuring the resistance of a circuit to ground or 



■^ C> 01 ± oj ro ^ 

-Jn < ffl 3> 3 




644 AMERICAN TELEPHONE PRACTICE. 

of any other circuit by means of the volt-meter, the cord circuit of 
Fig. 472 has been redrawn in part in Fig. 473, all parts not directly 
concerned in volt-meter tests being omitted. 

In making tests for grounds, throw the volt-meter key, No. 4, leav- 
ing the other keys in their normal positions. This will place the 
40-volt coil of the meter in a circuit between the ground and the 
ring side of the line under test, the circuit including the 40-volt 
battery. If a ground exists on the sleeve side of the line, the volt- 
meter will be deflected, owing to the passage of current through it. 
The same test may be made with respect to the tip side of the line 
by throwing the reversing key. 

The resistance of the ground may be measured by noting the 
amount of deflection. In the arrangement shown, the 40-volt coil of 
the volt-meter has a resistance of 100,000 ohms, and the 4-volt coil, 
10,000 ohms. The shunting key, when closed, places a shunt of 
1,1 1 1 ohms around the 4-volt coil, thus reducing the effectiveness 
of the volt-meter coil to one-tenth its original value. The volt- 
meter, therefore, has three effective resistances, 100,000, 10,000 and 
1,000, the first adapted to use with the 40-volt battery, and the other 
two for use with the 4-volt battery. 

If there is no external resistance in circuit with the volt-meter, 
the needle should show a full scale deflection when the 40-volt bat- 
tery is connected across the terminals of the 40-volt winding, when 
the 4-volt battery is connected across the terminals of the 4-volt 
winding, or across the 4-volt winding in parallel with the 1,1 11 ohm 
shunt coil. 

When the voltage is' such as to give a full scale deflection with no 
external resistance, and the zero position of the needle is at the left- 
hand end of the scale, then when the needle is deflected by current 
through the external resistance the external resistance is to the 
volt-meter resistance as the scale reading to the right of the needle 
is to the scale reading to the left. 

To determine then the value of the unknown resistance, it is only 
necessary to consider that the scale divisions from the zero point 
to the needle in its deflected position represent the volt-meter re- 
sistance, while the scale divisions from the needle to the point at 
which the needle comes to rest when the volt-meter is connected 
directly across the terminals of the battery, will represent the exter- 
nal resistance. For instance, if the volt-meter needle comes to rest 
at a point one-quarter of the way across the scale, the remainder of 
the scale (three-quarters) will represent the value of the unknown 



WIRE CHIEF'S EQUIPMENT. 



645 



resistance; hence, if the ioo,ooo-ohm winding has been employed, 
the unknown resistance will be 300,000 ohms; if the 10,000-ohm 
winding has been employed, it will be 30,000 ohms; and if the 
10,000-ohm winding has been shunted by the 1,111-ohm resistance 
coil the unknown resistance will be 3,000 ohms. 

It is evident by referring to Fig. 473 that leaving all keys except 
the volt-meter key in their normal positions, the 100,000-ohm coil 
and the 40-volt battery will be brought into service. Leaving the 
volt-meter key No. 4 thrown and throwing the battery switching 

-^. VOLTMETER 




FIG. 473.— VOLTMETER CONNECTIONS OF TESTING CIRCUIT. 



key No. 9, brings the 4- volt battery with the 10,000-ohm volt-meter 
coil into circuit. Throwing these two keys together with the shunt 
key leaves the same combination in service, and shunts the volt- 
meter scale. For greatest accuracy the volt-meter combination to 
be used in making a resistance determination should be that which 
gives a volt-meter resistance nearest to the resistance to be de- 
termined. 

The grounded or non-grounded condition of a line should not 
be decided finally by means of a volt-meter test only; for, if the 
ground happens to be of an intermittent character the swing of the 
volt-meter needle would be a most unreliable indication and might 
in extreme cases be scarcely perceptible. In that event the relay 



646 AMERICAN TELEPHONE PRACTICE. 

and sounder should also be used. This has the advantage that the 
comparatively large amount of current flowing may be great enough 
to permanently break down the insulation at the fault and make the 
ground of a permanent character. 

The method of testing for a ground with a relay and sounder may 
best be described in connection with Fig. 472. By throwing the 
relay key No. 5 and leaving all of the other keys in their normal 
positions, a ground on the sleeve side of the line will cause the relay 
to become energized and the sounder to be operated. If, now, the 
reversing key, No. 3, be thrown, this will test for a ground on the 
other side of the line. Tests for grounds with the relay depend, of 
course, on the resistance of the ground being low enough to allow 
current to flow through it to operate the relay. By testing the 
relay over a known resistance the greatest resistance through which 
it will be operated may be determined. With this resistance known, 
a line which tests clear under a relay test will be known to have no 
ground on it of a less resistance than that through which the re- 
lay will operate. 

Continuity tests of a line may be made as in testing for grounds, 
the only difference being that the subscriber's receiver must be off 
its hook and that a ground must be applied to the return side of the 
circuit by throwing the grounding key, No. 3. If there is no break 
in the circuit, the volt-meter will show a deflection. Of course, the 
fact that the subscriber's line is normally open to continuous cur- 
rents at the sub-station must be taken into account in making this 
test. A test for the continuity of a circuit should always be pre- 
ceded by a test for grounds, for if the line is grounded it might be 
broken on the other side of the ground, and still give the proper 
test for continuity. A good order, therefore, in making these tests 
in routine work is to first throw the volt-meter key, then the re- 
versing key, and then, if no deflection occurs due to a ground on 
the other side of the line, throw the grounding key. 

As the resistance of a line is usually comparatively low with 
respect to the coils of the volt-meter, continuity tests should as a 
rule be made with the low reading scale of the volt-meter using the 
shunt. If it is seen that the reading will not go beyond the range 
of the instrument, the low reading scale without the shunt or the 
high reading scale may be used. Of course, continuity tests may 
also be made with the relay. 

In making tests for crosses between two lines, the grounding cord 
shown in Fig. 474 is useful, and a few of these cords are usually 



WIRE CHIEF'S EQUIPMENT. 



647 



added to the wire chief's equipment. If the two lines are free from 
grounds, a cross between them may be determined by grounding 
one of them with the grounding cord, and testing for a correspond- 
ing ground on the other. 

To test for a cross or a short circuit between two limbs of a line, 
it is desirable to know the normal resistance of the line. It is well, 
therefore, to have a record of the resistance of each line when it is 
in good condition, and when the receiver is off the hook. This 
record should be kept in a card index, and any subsequent variation 
from the recorded resistance of a line should be taken as an indica- 
tion of trouble. 

The lifting of a subscriber's receiver off its hook sometimes gives 
an indication which is mistaken for an actual cross on the line. A 
cross on the line is usually of a non-inductive nature, while the 
path between the two limbs of the line through the subscriber's 
talking apparatus contains a considerable amount of retardation. 




FIG. 474.— GROUNDING CORD. 

This difference may often be made use of to determine whether or 
not the indicated cross is due to the subscriber's receiver being off 
its hook. If the ringing current is applied to the line by means of 
key No. I, an audible discharge due to the self-induction of the 
circuit will take place when the key is released, when the subscrib- 
er's receiver is off its hook. If the subscriber's receiver is on its 
hook, and the indicated cross is due to another cause, this discharge 
will not take place. Another means of determining whether or not 
the cross is due to the subscriber's receiver being off its hook may 
be made by depressing the circuit-opening button on the testing 
trunk (Fig. n), thus lighting the line lamp. The operator will 
respond and an attempt at conversation will give some indication 
as to the condition of the line, for if the trouble is in the form of a 
non-inductive cross, conversation will be scarcely audible. 

Ballistic methods of testing involve the reading of a sudden throw 
of the volt-meter needle, due to a momentary rush of current as 



648 AMERICAN TELEPHONE PRACTICE. 

from the discharge of a condenser or the kick of an impedance coil. 
Ballistic methods are used by the wire chief in those tests involving 
the capacity of the line and attached condensers. To test an ordi- 
nary one-party, metallic circuit, common battery line, first throw 
the volt-meter key, No. 4, then the grounding key, 3 ; next throw the 
reversing key quickly back and forth, causing a throw of the needle 
at each reversal. 

Consideration of Fig. 473 will show that at the closing of the volt- 
meter key a rush of current will take place from the live pole of the 
40-volt battery, through the 40-volt coil of the meter to the ring 
side of the line. This side of the line will thus receive a negative 
charge, while the tip side of the line will receive by induction a posi- 
tive charge. When the reversing key is thrown, the tip side of the 
line will be connected to the volt-meter and a current will flow from 
the battery through the volt-meter until- the charge on this side of 
the line is changed from positive to negative. There is naturally, 
therefore, a double discharge at each reversal, it being necessary for 
the current flowing through the* volt-meter to neutralize one charge 
and build up one in the opposite direction. The momentary rushes 
of current through the volt-meter coil will be in the same direction, 
and the deflection of the needle may thus be readily noted. By 
comparison with the double discharge from a known capacity at a 
known voltage the capacity of the line as indicated by. these read- 
ings may be readily determined. 

In case of very large capacities the 4-volt scale without the shunt 
should be used. 

When the extension bell is used in connection with a subscriber's 
line, it may be connected either in parallel with the bell associated 
with the subscriber's set, or a separate condenser may be placed 
in series with the extension bell, and bridged directly across the line. 
The former arrangement will have no appreciable effect on the 
capacity of the line, while the latter arrangement will increase it 
considerably. 

Party lines having alternating current relays or magneto bells 
bridged across the line in series with condensers, will give an in- 
creased throw for each added instrument. Party lines having the 
bells connected to ground on either side of the line will give a throw 
for the bells on the ring side of the line when the reversing key 
returns into its normal position, and a throw for the bells on the tip 
side of the line, when the reversing key is thrown out of its normal 
position. In case the bell is connected from one side of the line 



WIRE CHIEF'S EQUIPMENT. 649 

to ground through a condenser, the throw of the needle of the volt- 
meter, when a ballistic test is being made, will be equal to about one- 
half of the throw occasioned when the bell and condenser are bridged 
directly across the line. This is due to the fact that in the former case 
the charging current alone is measured, while in the latter case, 
both the charging and the discharging currents are measured. 

The distance from the exchange of a break in a long line which 
has a high insulation resistance may be approximately located by 
observations taken of the throw of the volt-meter needle. 

It is often necessary to determine the continuity of a circuit which 
contains a condenser, thus rendering continuous current methods 
unavailable. To make a test of this character throw the continuous 
ringing key, No. 6 (Fig. 472), and if necessary, the reversing key. 
The continuity of the circuit for alternating currents may thus be 
determined by connecting the ends of the circuit beyond the con- 
denser through a magneto bell. This method may also be applied 
when the two ends of a circuit are separated by a repeating coil. 

Tests for foreign currents which may be present, due to a cross 
between some other line and a telephone line, may be made by throw- 
ing the battery cut-off key, No. 8, and the volt-meter key, No. 4. 
This bridges the 40-volt coil of the volt-meter across the line to 
measure the voltage present. The grounding key, No. 3, and the 
reversing key, No. 2, may be used as occasion demands. 

The tests for switch-board troubles are perhaps the most varied 
of all, and methods may be often devised to meet the particular case 
in hand. A convenient way of locating breaks in the multiple wiring 
is to employ an ordinary head telephone attached to a switch-board 
plug, which may be inserted into the jacks of the line undergoing 
test at the multiple switch-board. With the Western Electric line 
circuit, or any other circuits, where the tip and ring conductors of 
the jack are permanently in contact with the tip and sleeve sides of 
the line, the continuity of the circuit to any jack will be indicated by 
the lighting of the line lamp when the plug is inserted in that jack. 
As soon as the plug is inserted in a jack beyond the break, the line 
lamp will not light, and the break may therefore be located between 
the last jack through which the lamp will be lighted and the first 
one through which it will not. 

In those boards, as for instance those of the Kellogg and Strom- 
berg Companies, where the line jacks are not normally connected 
with the line, a similar test may be made by placing a battery and 
a buzzer across the tip and sleeve strands of the multiple at the 



650 AMERICAN TELEPHONE PRACTICE. 

first section of the board, and then plugging into the successive jacks 
of that line with a receiver bridged across a plug. The noise will 
not be heard, except perhaps faintly, beyond the break. 

Under the heading of miscellaneous tests, almost an infinite variety 
might be mentioned, these varying in accordance with the nature of 
the trouble, the nature of the circuit and apparatus, and the ingenuity 
of the person making the test. 



CHAPTER XXXIV. 

THE LAY-OUT AND WIRING OF CENTRAL OFFICE 
EQUIPMENTS. 

A visit to a modern central office containing a large common 
battery multiple switch-board, is apt to impress one with the fact that 
the wiring of a central office equipment is one of the fine arts. On 
account of the great multiplicity of circuits, the systematic and 
neat arrangement of the various wires connecting the different pieces 
of apparatus in their proper circuit relations is of great importance, 
and the really artistic work which is seen in a large central office 
is not the result of a desire to merely "make things look pretty," but 
is brought about by necessity. 

It is not unusual in any line of work that good workmanship, 
when applied in carrying out good design from the standpoint of 
utility alone, will result in what will be called an artistic product. 
Nowhere is this more apparent than in the interior arrangement of a 
telephone central office. 

The function of the wiring is, of course, to connect the different 
parts of apparatus in proper working relation. It is necessary, for 
several reasons, to place these various pieces of apparatus in different 
parts of the office, in accordance with a certain classification gov- 
erned largely by the functions each class of apparatus is to perform. 
Thus all those parts necessarily within reach of the operators, such as 
signals, jacks, plugs, keys and the like, are placed in the operating 
room, and form, when assembled with their proper mounting, the 
switch-board proper. Those parts individual to the line circuits, 
including line terminals, arresters, relays and distributing means, 
are placed on racks in a separate room or rooms and arranged with 
particular reference to convenience in maintenance and repairs. 
Those parts concerned in the generation or control of the various 
currents necessary for the operation of the exchange form another 
distinct group, known, as a whole, as the power plant. 

While all of these parts must be arranged primarily with regard 
to the most efficient performance of their various functions and to 
convenience of maintenance, proper regard must be had in the lay- 
out to the securing of an economical arrangement with regard to 

651 



652 AMERICAN TELEPHONE PRACTICE. 

the wiring, extending between the various pieces of apparatus in 
each class, and between those in the different classes. This matter 
deserves much careful attention from the standpoint of first cost, 
as the cable and wire is in itself expensive, and much may be saved 
by a proper relative location of apparatus of the different classes. 

In order to make clear the arrangements necessary from the wir- 
ing standpoint, the wiring of a line circuit will first be considered. 
The line circuit of the Western Electric Company, shown in Fig. 
264, will be taken as an example. The circuit of one line, as actually 
wired, is shown in Fig. 475, this being an elaboration of the con- 
ventional circuit shown in Fig. 264. Such a circuit as this is usually 
called a wiring diagram, because in it the various connections, as 
they actually exist in practice, are indicated, together with other 
information as to the makeup of the cables, sizes of wire, location of 
the apparatus, etc. Taking this circuit as a basis, the progress of a 
line will be traced from the place where the lines enter the office 
building through the two distributing frames to the switch-board 
and other portions of the equipment. 

It will be seen that the line after entering the office building, is 
first led to the line side of the main distributing frame, and is there 
connected by means of duplex jumper wire to the arresters on the 
switch-board side of the main distributing frame. From this the 
two sides of the line continue to two of the three clips on the line or 
multiple side of the intermediate distributing frame. Up to this 
point in the circuit shown, all wires have been in twisted pairs, there 
being only two wires to each circuit. There are, however, three clips 
on the line side of the intermediate frame, and from these three 
lead three wires to the three contacts on the multiple jacks, two 
of these being simply a continuation of the tip and sleeve conductors 
of the line, the third being a lead to the test rings on all the jacks. 
The two line wires take the names "tip" and "ring," and the third 
wire the name "sleeve," these being derived from their plug con- 
tacts ; hence the designation of their various distributing frame ter- 
minals, "T," "R" "S." 

The wires leading from the main to the intermediate distributing 
board are usually grouped in 21-pair cables, 20 pairs being the 
unit of wiring throughout the exchange from the arrester side of 
the main distributing frame. The cables leading from the main to 
the intermediate distributing frames are termed the "main to inter- 
mediate" cables. 

In order to accommodate the three wires leading from the inter- 




653 



654 AMERICAN TELEPHONE PRACTICE. 

mediate distributing frame to the first multiple section of the switch- 
board, the wires serving the 20 lines are grouped into a 63-wire 
cable, this cable consisting of 21 twisted pairs serving the tip and 
ring contacts of the jacks, and 21 single wires serving the sleeve 
contacts. These are the "intermediate to multiple" cables, some- 
times called "long multiple cables." 

On the opposite side of the intermediate distributing frame the 
conditions are more complex. All of the relays associated with the 
lines are mounted on what is called a relay rack, and from the four 
clips on the answering jack side of the intermediate distributing 
frame to the relay rack, four individual conductors for each line 
extend. One of these is that which connects the line lamp clip 
L, on the intermediate frame with the lamp contact of the line relay. 
Two others connect the tip and ring contacts, T and R, respectively, 
on the frame with the long contact springs of the cut-off relay. The 
fourth connects the sleeve of the answering jack and those of the 
multiple jacks to the coil of the cut-off relay. These four wires, 
serving 20 lines, are bunched into a cable, forming with four spare 
wires, an 84-wire cable. This cable is termed "the intermediate 
to relay rack cable." The four wires belonging to any line in this 
cable consist in each case of a twisted pair, "tip" and "ring," and 
of two straight wires, "sleeve" and "lamp." 

Another group of cables extends from the same clips on the 
answering jack side of the intermediate frame to the answering 
jacks, and these are termed "intermediate to answering jack cables." 
There are also four wires in this cable for each line, these being the 
wires extending to the three contacts in the jack and to the lamp. 
These as in the intermediate to relay rack cables consist in each case 
of a twisted pair and two straight wires. 

Besides the cables so far considered there are the "short multiples" 
leading from section to section of the switch-board, each cable con- 
necting a strip of multiple jacks in one section with the correspond- 
ing strip in the next. There are therefore for each line one less 
short multiple cables than there are sections of the switch-board. 

In a three-wire system like that of the Western Electric Company 
under consideration, the cable in the short multiples consists of 
63 wires in all, 21 pairs and 21 individual wires. 

Various leads, such as those extending from the live side of the 
battery or from ground are made of okonite or similar wire of suf- 
ficient carrying capacity to supply the maximum current demanded. 
It is common practice to run the principal common battery and 



CENTRAL OFFICE EQUIPMENTS. 655 

ground wires bare, unless the particular place in which they are used 
makes it safer to insulate them. Much of the common wiring is 
shown in the diagram of a power plant, Fig. 397. 

In order to facilitate the work of properly forming the cables and 
bringing the corresponding wires out at such points as to reach 
the corresponding terminals at the two ends of the cable, the wires 
in the cable are arranged in accordance with a certain color scheme, 
or, as it is more often called, a color code. The outer wrapper of 
cotton of the different wires is, with this end in view, dyed so as to be 
of a distinctive color. The colors by which the various pairs of 
wires in a 21 -pair cable are distinguished, are composed of the fol- 
lowing, always used in the same order : Blue, orange, green, brown 
and slate. One wire of each pair always has its cotton covering 
formed of one of these colors, or a combination of two or three of 




FIG. 476.— METHOD OF FANNING OUT CABLES. 

them. Such a wire when forming part of a pair usually has a white 
mate. When there are three wires associated together, as in a 63- 
wire cable, the third has the same code as the colored wire in 
the pair, but also has a thread of red running with the other colors 
in each case. The color code for a 21 -pair cable, therefore, is as 
follows : Blue, orange, green, brown, slate, blue-white, blue-orange, 
blue-green, blue-brown, blue-slate, orange-white, orange-green, 
orange-brown, orange-slate, green-white, green-brown, green-slate, 
brown-white, brown-slate, slate-white. The spare wire in a twisted 
pair is solid red, its mate being solid white. When there is a third 
wire associated with the spare pair it is red- white-blue. 

The method of forming the end of a cable so as to facilitate 
the soldering of its component wires is shown in Fig. 476. This 
figure applies to machine cables, that is, to cables made up by machin- 



656 



AMERICAN TELEPHONE PRACTICE. 



ery in continuous lengths, and afterward cut to length before form- 
ing. Each cable is stripped at its end, and butted with tape at the 
point where the covering of the cable ends. The wires leading from 
the cable are then formed and laced in the manner shown in Fig. 
476, the various pairs being led out at intervals corresponding to 
the intervals between the terminals in the strip to which they are 
to be attached. 

The method of wiring the multiple cables is shown in Fig. 477, 
which shows a single strip of jacks with the ends of the cables 
leading to the two adjacent sections soldered to it. A two-point 
jack only is shown in the figure. 

The lay-out of a telephone exchange building involves not only 
the floor plan of the operating room, of the terminal room, and of 




FIG. 477.— MULTIPLE JACK WIRING. 



rooms occupied by other portions of the equipment, but in modern 
practice the primary design of the building itself. It is a matter 
for which few general rules may be laid down, it being necessary 
to solve the various problems arising, in view of the conditions in 
each case. It may be said, however, in general, that except in 
comparatively rare cases, the space available for an operating room 
usually makes it desirable to arrange the board in horseshoe form, 
the various sections being arranged in a line facing inward around 
the operating room. The reasons for facing the boards inward 
instead of outward are several. The direct light from the windows 
does not then fall on the face of the board, which might prevent 
the lamp signals from being distinctly seen by the operator when 
lighted. Moreover, the light from the windows in the walls of the 



CENTRAL OFFICE EQUIPMENTS. 657 

room may shine behind the board, which is an advantage from the 
standpoint of repairs. When the operators are on the inside rather 
than the outside of the curve of the board, they are within sight of 
the chief operator and other officials of the operating room. The 
operators at the angles of the boards are, by reason of being within 
the angle, enabled to reach the entire section of multiple jacks, 
which would not be the case were they on the outside of the curve. 
Lastly, there is a distinct advantage from a constructive standpoint 
in that more room is afforded at the boards in which to make the 
multiple cable connections when the cables are on the outside rather 
than the inside of the curve. 

As a rule, all cables entering the switchboard, whether multiple, 
answering jack or other, are led into one end of the board through 
what is usually called the turning section, the cables being led up 
from beneath or down from above, usually the former, and assuming 
a horizontal direction after passing through what is called a cable 
support, within the turning section. Most of these cables run from 
the end of the intermediate distributing frame, and it is therefore 
obvious that the end of the intermediate distributing frame should 
be as close to the turning section of the multiple board as possible, 
in order to save in the amount of cable required between them. 

It is also important that the arrangement of the distributing 
frames and the relay rack be such that a minimum amount of cable 
shall be required in the main to intermediate and in the intermediate 
to relay rack runs. The length of these two runs, however, is not 
of as much importance as that of the run between the intermediate 
frame and the turning section on the switch-board, because this run 
always includes the intermediate to multiple cables and usually also 
the intermediate to answering jack cables. 

A point of not less importance is that the distance from the main 
cable vault of the office to the main frame should be as small as 
possible, so as to shorten the run of the street cables to the main 
frame. In modern construction these are usually lead covered and 
therefore more expensive. 

It is necessary, especially in large exchanges, to provide separate 
room for the storage batteries for the purpose of isolating them 
from all other portions of the equipment, particularly on account 
of their fumes. Such a room should always be provided with ample 
ventilation, particularly with a vent near the top of the room, and 
if necessary, with forced draft. The room should, moreover, be 
light, in order to afford ready means for inspection of the plates. A 



658 AMERICAN TELEPHONE PRACTICE. 

concrete floor is perhaps most common in the battery room, but sev- 
eral exchanges have been installed using a rough glazed tile floor, 
with the binding between the tiles resembling tar. Such a floor has 
the advantage of tending to prevent the workmen from slipping 
when handling the acid and also of not being attacked by the acid 
when spilled upon it. 

The power plant should be located as close to the battery room 
as possible, and preferably in the terminal room containing the dis- 
tributing frames and relay racks. 

The wire chief's desk is usually placed in the terminal room, and 
where this room also contains the power apparatus, the supervision 
of the wire chief may extend over this portion of the apparatus as 
well. 

There is still some controversy in regard to the best method of. 
arranging the cables and line terminal apparatus between the cable 
entrance and the switch-board apparatus in central station offices. 
Two general methods are now practiced, either of which is subject 
to many modifications. One of these consists in terminating the 
street cables in pot-heads or their equivalents, and running directly 
from these to line clips on the main distributing frame, the switch- 
board side of this frame carrying the arresters. From the arresters 
the various cables run to the relay rack or intermediate distrib- 
uting frame, and thence to the switch-board proper. The other 
method is to terminate the cables in iron terminal heads placed on 
a separate rack, these terminal heads carrying the arresters, from 
which cables then run to the main distributing frame and thence to 
the intermediate frame and relay rack, and to the switch-board. 

The former method, that is, of terminating the street cables in pot- 
heads or their equivalent devices and of placing the arresters on the 
switch-board side of the main distributing frame, is growing in 
favor and has been the practice of most of the Bell operating com- 
panies. Some of the advantages of this method may be outlined 
as follows: 

First, it brings about a considerable saving of expense over the 
construction using the iron terminal heads. This is so, principally 
because where the pot-head construction is used the number of ar- 
resters provided need be equal only to the number of lines in use on 
the switch-board; rather than equal to the number of. cable pairs in 
the street cables, as may be required where the iron terminals are 
used. The number of pairs in the outside cables is usually greatly 
in excess of the actual working capacity of the switch-board, and 



CENTRAL OFFICE EQUIPMENTS. 659 

the providing of an arrester for every pair in the street cables in- 
volves a considerably greater investment than would be required 
where only one for each switch-board line is needed. Further- 
more, the price of the iron cable heads with arresters is considerably 
more than the price of the arresters without the heads, and there is 
an additional cost in the rack upon which the iron terminals are to 
be mounted, as well as in the additional amount of wiring and sol- 
dering to be done where the iron terminals are used. This addi- 
tional wiring and soldering is brought about by the fact that cables 
must be led to the iron arresters to which they are soldered, and 
other cables led from the arresters to the line side of the distributing 
frame; whereas, in the case where the iron terminals are not used, 
the cabling is carried directly from the street cables to the line side 
of the main switch-board. 

Second, the method of using pot-heads requires less floor space 
than the other, no cable rack being required. 

Third, it is more convenient to associate the arrester with the 
switch-board line than it is to associate it with the outside line. 
When the arrester is mounted directly on the switch-board side of 
the main distributing frame it may be said to become a part of the 
line circuit of the switch-board, and in the exchange is associated 
definitely with the particular line and cut-off relay, as well as cer- 
tain other portions of the apparatus on the main switch-board. 
When, however, the arrester is mounted on a separate cable head 
outside of the main distributing frame, it becomes more properly a 
part of the line equipment, inasmuch as any arrester must then 
always be associated with any particular line. Inasmuch as the 
arrester is a part of the central office apparatus, it stems better to 
consider it as belonging to a certain switch-board line rather than 
to an outside line. This is particularly true in case of trouble need- 
ing quick action, for, when the arrester is mounted on the switch- 
board side of the distributing frame, no jumper list need be con- 
sulted to get at the arrester belonging to any switch-board line. 

Fourth, the iron cables are supposed to hermetically seal the lead- 
covered cables entering the office. They are frequently not mois- 
ture-proof, however, and are liable to be left open at any time by 
workmen. Furthermore, the interior arrangement of the conduc- 
tors in these heads whereby the wires in the cables are attached to 
the wires in the switch-board cable, is always extremely awkward. 
because it is necessarily crowded. Pot-heads, when properly made. 
afford a much better protection to the cable insulation than can 



660 AMERICAN TELEPHONE PRACTICE. 

with certainty be afforded with the iron cable heads. These latter 
are always necessarily punctured full of holes through which the ter- 
minals must pass, and therefore depend for their air-tight condition 
upon the successful plugging of these holes with the rubber in- 
sulation. 

On the other hand, there are several features which may be cited 
as disadvantages in connection with the practice of terminating the 
cables in pot-heads or equivalent devices and carrying the arresters 
on the main distributing frame. 

There is a considerable amount of soldering always necessary at 
the distributing frames due to the constant changes of the jumper 
wires. From this viewpoint the placing of the arresters on the dis- 
tributing frame is disadvantageous in that drops of solder are likely 
to fall down through the arrester strips and short-circuit some of 
the springs or connectors. The arresters are necessarily closely 
bunched, and a drop of solder falling through them may introduce 
several cases of trouble in different lines. This is not as likely to 
happen where the arresters are mounted on the outside of cast-iron 
terminal boxes, because in this case most or all of the soldering is 
necessarily done on the inside of these boxes. 

Another disadvantage often pointed out by advocates of the iron 
terminal head is that when the arresters are mounted on the switch- 
board side of the distributing frame no protection is afforded for th: 
line side of the distributing frame or for the jumper wires. This, 
in the old methods of construction where beeswaxed or paraffined 
jumper wires were used, and where the distributing frame was built 
largely of wood, w r as a very serious drawback. Now, however, it 
is of less importance, because the use of flameproof jumper wire and 
structural iron frameworks has rendered the liability to damage by 
fire in the distributing frame itself very slight. 

This brings us logically to still another method of construction 
which in some circles is growing in favor. In this pot-heads or 
some equivalents are used for protecting the outside cable insula- 
tion, and the cables from these are terminated in arresters on a sepa- 
rate arrester*frame. In this case the arresters are mounted on iron 
bars and are entirely open, as when mounted on the distributing 
frame. From the arresters the cables then lead to the line side of 
the distributing frame. The switch-board side of the distributing 
frame in this case consists only of ordinary clips for connecting the 
cables leading to the intermediate and relay racks and the switch- 
board. 




CC1 




C62 



CENTRAL OFFICE EQUIPMENTS. 665 

By this construction protection is afforded to the whole of the 
main distributing frame and the cables leading to it, as in the case 
where the arresters are mounted on iron terminal heads. Also the 
advantages concerning the use of solder are secured. The running 
of the jumper wires on the main distributing frame is made some- 
what more convenient, because of the absence of the arrester strips, 
which are often of such nature as to somewhat crowd the work of 
soldering the jumper wires. 

Of course, this latter method has the disadvantage of requiring 
more arresters — that is, one for every street conductor. It also 
requires more room on account of the extra rack for mounting the 
arresters. 

Although each of the three methods has its advocates among 
competent engineers, the consensus of opinion seems to be strongly 
in favor of the method wherein the arresters are carried on the 
switch-board side of the main distributing frame. 

Practice differs to a considerable extent when this latter arrange- 
ment is used as to the method of connecting between the street 
cables and the line side of the main distributing frame. One 
method, and under certain circumstances a good one, is to end the 
street cables within the exchange in pot-heads, running from these 
pot-heads small regular switch-board cables to the main distributing 
frame. It is common in this case to use 20-pair switch-board cable, 
and where the condition of the portions of the building through 
which these cables are to pass is such as to be depended upon for 
absolute dryness there seems to be no reason why this cable should 
be lead-covered. They are, however, frequently lead-covered, al- 
though when this is done larger cables than 20-pairs are com- 
monly used. The dividing up into small cables at the pot-head is 
of distinct advantage in distributing the wires of the large cables 
properly to the terminals on the distributing frame, and is to be 
preferred to bringing the larger cable to the distributing frame and 
having to split it up so as to accommodate it to several strips. 

Another method, and one widely used, is to use in the place of 
pot-heads a considerable length of wool-insulated cable, lead-cov- 
ered, this being spliced directly onto the street cable and serving in 
itself to keep moisture out of this cable. 

In the Western Electric system (see Figs. 438 and 475) the ar- 
rangement of the line circuit is such that the cables naturally pass 
directly from the main frame to the intermediate and thence to the 
relay rack. In the Kellogg system (sec Fig. 459) the cables from 



€64 AMERICAN TELEPHONE PRACTICE. 

the main frame pass first to the relay rack and then to the inter- 
mediate frame. In plants installed by the Western Electric Com- 
pany, therefore, the frames are arranged on the floor in this order, 
main distributing frame, intermediate distributing frame and relay 
rack. In Kellogg plants the order is main frame, relay rack and 
intermediate frame. In all cases, however, all of the cables in- 
volving the line circuits pass directly from the intermediate frame 
to the switch-board. 

The cables leading from one frame to another, or from the inter- 
mediate frame to the switch-board, are usually carried in iron chan- 
nels or runways secured either to the frames themselves or to the 
building. The cables are piled in the runways in systematic fashion, 
thus economizing space and rendering them accessible for repairs. 

In the Western Electric plants the cables leading from one rack 
or frame to another lie in runways extending between the ends of 
the racks, whether the racks are mounted side by side or end to end. 
Thus, in the case of the intermediate to relay rack cables, they are 
run in a trough extending along the upper side of the intermediate 
frame to one end of this frame, thence across to the top of the relay 
rack and down its entire length. All cables were thus led through 
this trough at the end of the two racks, requiring much more cable 
than would be necessary if the cables were laid straight across from 
the intermediate frame to the relay rack in case the frames were 
mounted side by side. 

In the later plants of the Kellogg Switch-Board and Supply Com- 
pany the three frames are mounted side by side, the relay rack being 
between the main and intermediate frames. This construction in- 
volves the placing of a horizontal lattice-work extending between 
the top of the main frame and the top of the intermediate frame, 
upon which all cables extending between the three frames may be 
led in the most direct manner. In order to prevent the criss- 
crossing and consequent tangling of the cables on this horizontal 
rack, the distributing frames are so proportioned with respect to 
each other that a bay of relays will be made just as wide as the in- 
terval occupied by three arrester strips on the main frame, and this 
bay will be provided with the same number of relays as there are 
arresters on a corresponding space on the main frame. This makes 
it possible to extend all cables straight across between the frames, 
thus securing a systematic arrangement without piling up the cables 
in a compact mass. The open construction thus secured is advan- 
tageous in case of repairs or changes, while the saving of cable is 
enormous. 



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6C6 



CENTRAL OFFICE EQUIPMENTS. 667 

Such construction is well shown in Figs. 478 to 481, inclusive, 
these being views of the terminal room in one of the large offices of 
the Keystone Telephone Company, of Philadelphia, Pa. This work 
is that of the Kellogg Switch-Board and Supply Company. 

Fig. 478 shows a view of the terminal (line and switch-board) side 
of the main distributing frame. The charging machines, power 
switch-board and wire chief's desk are also plainly shown. Fig. 
479 is a view taken in the opposite direction from that of Fig. 478, 
showing the jumper side of the main frame and the relay side of the 
relay rack. The other side of the relay rack, upon which all wiring 
is done, is shown in Fig. 480, this view showing on the right the 
jumper side of the intermediate frame. 

The general overhead construction is shown quite clearly in Fig. 
481, which is a view of the tops of the main frame, relay rack and 
intermediate frame. 

In all modern common battery multiple systems the main body 
of cables leading to the switch-board extend from one end of the 
intermediate distributing frame in an iron runway between the end 
of this frame and the turning section of the multiple board. 

One end of such a cable run is shown in Fig. 482, which is a view 
of the end of the intermediate distributing frame of the new Cort- 
landt Street exchange of the New York Telephone Company. The 
horizontal or multiple side of this frame is that toward the observer 
in this view, and from this side the cables lead through the diagon- 
ally mounted runway, as shown, to a horizontally disposed runway 
which passes to the turning section at the end of the multiple board. 
From the vertical or answering side of this frame lead the interme- 
diate-to-answering-jack cables, and these pass into another hori- 
zontal runway, where they may be seen in the upper left-hand corner 
of Fig. 482. The other end of these horizontal cable runs is shown 
in Fig. 483. This view shows the rear of the turning sections of 
both the subscriber's multiple board and the incoming trunk boards. 
The multiple cables to these switch-boards turn right and left 
through ninety degrees in a horizontal plane, and then downward 
into the turning sections at the ends of their respective boards, as 
shown. Behind the panel work shown in the lower part of this fig- 
ure the multiple cables again turn into a horizontal direction into the 
switch-boards behind the multiple jacks. 

The method of arranging the multiple cables always involves 
serious attention. The practice is now almost universal to lay the 
multiple cables on a shelf directly back of the jacks which they 



668 AMERICAN TELEPHONE PRACTICE. 

serve, the thickness of the cables being so arranged that they will 
build or pile upon each other at the same rate as the strips of jacks 
in the face of the board; thus, the cable serving any particular strip 



FIG. 482.— END OF INTERMEDIATE DISTRIBUTING FRAME— CORTLANDT 

STREET OFFICE. 

of jacks will always lie on the same level as that strip of jacks. It 
is evident that if the cables are thicker than the jacks so that they 
will build up more rapidly, the available space for the cables will 



CENTRAL OFFICE EQUIPMENTS. 



669 



be filled before all the jacks required by the ultimate capacity of the 
exchange are put in place. Furthermore, the "skinners," as the 
individual wires leading from the cables to the terminals of the jacks 




FIG. 4b3.— TURNING SECTION— CuRTLANDT STREET OFFICE. 



are called, would have to be longer than would be necessary where 
the cables build up at the same rate as the jacks. 

A rear view of a portion of the large multiple board of the Cort- 
landt Street exchange of the New York Telephone Company is 



670 



AMERICAN TELEPHONE PRACTICE. 



shown in Fig. 484. This shows the appearance of the multiple cable 
when piled in place back of the multiple jacks. 

In Fig. 485 is shown an excellent rear view of the wiring of a 
common battery multiple board, this being that in the main office 




FIG. 484.— VIEW OF MULTIPLE CABLES— CORTLANDT STREET OFFICE. 



of the Bell Telephone Company at St. Louis, Mo. A general view 
of the front of this board was shown in Fig. 284. In the upper por- 
tion of Fig. 485 may be seen the mass of multiple cables connecting 
with the multiple jacks. Below these are the cord terminal jacks 




(171 



672 



AMERICAN TELEPHOXE PRACTICE. 



and supervisory relays, and at the bottom of the view is the mass 
of intermediate-to-answering cables, these cables lying in a running 
box on the floor, a portion of which is removed in this view for the 
purpose of giving a better view of the cables themselves. 

The pile of multiple cable is always as many cables high as there 
are horizontal rows of multiple jacks in the board. It is as many 
cables deep (front to rear) as there are panels in the switch-board 
section. From this it is evident that the skinners of the cables lead- 
ing to the jacks in the first panel of each section are the shortest, 
those leading to the second panel a little longer and so on to those 
leading to the last panel, which are longest. This is well shown in 
Fig. 486, which is a horizontal plan of the arrangement of jacks and 
cables in any one layer of the multiple. 




FIG. 4S6.-PLAN OF MULTIPLE CABLING. 



The forming up of the multiple cables and soldering them to their 
jacks should always be done in the factory. In this way a vast 
amount of soldering may be accomplished before the apparatus is 
shipped to its destination, and in fact, the multiple jacks and multiple 
cables should all be tested out for transpositions, crosses, open wires, 
etc., before they leave the factory. The premises in which a large 
multiple board is being installed is no place for the performance of 
the almost endless task of soldering the multiple cables. The work 
may be done much more cheaply in the factory, where skilled labor 
is kept on hand for that purpose. It is therefore difficult to see 
why the practice still exists in some quarters in shipping the cables 
in lengths to be cut up, formed and soldered on the premises in the 
installation. There is, moreover, no reason, if proper engineering 



TOP VIEW 




13 



FIG. 487.— DETAILS OF CABLE LAY-OUT. 
673 



674 



AMERICAN TELEPHONE PRACTICE. 



work is done in designing the wiring, why practically all of the 
cables, both long and short, should not be prepared, cut to length, 
formed, and in some cases soldered to their terminals in the factory 
rather than doing it at the installation. To do this, however, re- 
quires the utmost nicety in the laying out of the various details of 
the plant. 

The scheme of cabling down to the running of each individual 
cable must be worked out at the draughting table on as large a scale 
as possible, after which the various cable runs are scaled and cable 




FIG. 488.-PLAZA CENTRAL OFFICE BUILDING. 



cut to length and formed. Modern methods applied in this direc- 
tion permit practically all cable forming being done before the ap- 
paratus is shipped; even the longest cables being cut to length and 
formed to such a nicety as to fit exactly and without waste into the 
various cable runs in which they are to be placed. 

When it is considered that sometimes many hundred cables must 
be placed in a single runway, and that these cables must be piled in 
compact forms, a certain number in a layer and a certain number of 
layers deep; that they must often in going from room to room or 
from floor to floor be led around rather complex curves so that 



676 AMERICAN TELEPHONE PRACTICE. 

some will be bent on a short radius and some on a long, the nicety 
of detail in this work will be appreciated. A portion of a detail 
sheet showing the manner in which these cable plans are made is 
given in Fig. 487, this being taken from a drawing made for actual 
practice. 

As already stated the various problems relating to the lay-out of 
a large telephone central office equipment must be solved with re- 
gard to the conditions of each individual case. As it is impossible 
to discuss in the space available all of the factors in such a problem, 
this chapter will conclude with a brief description of two modern 
central office equipments which are thought to represent present good 
engineering practice. 

In Fig. 488 is shown a view of the Plaza central office building 
of the New York Telephone Company, this office having been put 
into commission about two years ago. This view, while showing 
the general appearance of the building, shows also much of its inte- 
rior arrangement by virtue of portions of the wall being cut away. 

A plan view of the second floor of this building is shown in Fig. 
489. This floor is devoted entirely to the distributing frames, the 
relay racks and the power plant. 

Fig. 490 shows a plan of the fourth floor, which is devoted entirely 
to the operating room containing both the subscribers' and trunking 
sections. 

Fig. 491 shows a plan of the third floor, which is devoted to the 
operators' quarters and offices. 

Referring to Fig. 488, the underground cables may be seen in 
the lower right hand corner of the picture, these entering the base- 
ment of the building from the subways. These cables are each led 
into the building and up to the second floor through an individual 
iron duct, thus doing away with the ordinary cable shaft for leading 
these cables from the cable vault to the terminal room. At about 
the floor line of the second floor the underground cables are con- 
nected, by means of pot-heads, to rubber covered cables leading to 
the distributing board. These pot-head joints are concealed under a 
false floor in that portion of the room. 

The doing away with the open cable shaft and substituting there- 
for the individual iron ducts is a great advantage from the stand- 
point of fire hazard, and also is advantageous from a mechanical 
standpoint, there being much less liability to damage the cables than 
there is with the use of an open cable shaft with its numerous cable 
clamps. These iron ducts are built into the wall of the building 




677 




678 



CENTRAL OFFICE EQUIPMENTS. 



679 



thus reducing still more the fire risk. In order to prevent the empty 
ducts from forming flues in case of fire, all unused ducts are plugged 
at both ends. 

The main distributing frame cannot be seen in Fig. 488, as it is 
behind the stairways in that figure. Its position, however, is well 
shown in the plan of 489. As will be seen from this floor plan the 
intermediate distributing frame is arranged at the end of the main 
frame; and the relay rack for the subscribers' lines runs alongside 
of the intermediate frame. The trunk relay and repeating coil rack 




FIG. 492.— FOYER-HOME TELEPHONE COMPANY, LOS ANGELES, CAL. 



are mounted in parallel lines with the subscribers' relay rack and 
intermediate frame. In a separate rack extending in a line parallel 
with the main distributing frame are mounted the service meters, 
these being used to a large extent in this exchange to determine the 
proper charge to the subscribers for service. 

The power plant, consisting of the power board, battery, two 
charging motor generators and three ringing machines, are arranged 
as clearly shown in this view, all of them being in the same room 
with the various terminal apparatus. 

The wire chief's desk is so located in the central portion of the 



680 



AMERICAN TELEPHONE PRACTICE. 



room as to afford from it a comparatively clear view of all parts of 
the apparatus on this floor. 

From the left hand end of the top of the intermediate distributing 
frame extends a cable shaft leading to the fourth floor or operating 
room. This runs up in the rear corner of the building, and is best 
shown in the plan view of the third floor in Fig. 491, where it may 
be seen between the operators' china closet and the chimney. All 
cables leading to the switch-board pass up through this shaft to the 
third floor, where they enter the turning sections, respectively, of 




FIG. 493.-CABLE VAULT-LOS ANGELES OFFICE. 



the subscribers' sections and the trunking sections, these two turn- 
ing sections being located adjacent to each other at the point where 
the subscribers' sections and the trunking sections come together at 
a right angle. 

The arrangement of the operating room needs little description, 
as it is clearly shown in the plan of Fig. 490. The capacity of this 
switch-board, as provided for in multiple jack space and in the 
number of sections, is 9,600 lines. As room is provided for only 
nineteen subscribers' sections, this indicates an average of about 
500 lines per section, which would be considered high in a city of 



CENTRAL OFFICE EQUIPMENTS. 



681 



the size of New York, save for the fact that this office serves to a 
great extent a residence district. 

The operating room has a hard-wood floor over which rubber 
tiling is laid, this being advantageous in point of the degree of 
quiet which it affords, and also in the ease with which it may be kept 
clean. 

In the lay-out of this building it would have been more economi- 
cal, from the standpoint of first cost, to have devoted the third floor 
to the operating room, and the fourth floor to the operators' quar- 
ters. By making the terminal as close to the ground as possible 




FICx. 494.-ENTRANCB TO CABLE SHAFT-LOS ANGELES OFFICE. 



and by placing the main distributing board as nearly in a line 
above the cable vault as possible, as has been done in this case, much 
is saved in length, and therefore in the cost of the lead-covered 
cable leading from the subways to the distributing frame. Bv plac- 
ing the operating room on the third floor, so as to be immediately 
above the terminal room a considerable amount would have been 
saved in the cables leading from the intermediate frame to the 
multiples and from the intermediate frame to the answering jacks. 
This latter advantage, however, was sacrificed, undoubtedly, because 
of the fact that the top floor of the building was more adaptable 



CENTRAL OFFICE EQUIPMENTS. 683 

to the purposes of the operating room on account of light, air and a 
greater degree of quiet. 

The floors of this building, as in the other late central office 
buildings of the New York Telephone Company, are designed to 
support a maximum load of 200 pounds to the square foot, this 
being about double that usually provided for in office buildings. 
The building is as nearly fire-proof as possible, the walls are of 
smooth finished cement, and instead of the ordinary wood trim- 
ming metal window frames and marble surbases are provided. 

One of the most complete telephone central office equipments 




FIG. 496.— VIEW OF DISTRIBUTING FRAMES AND RELAY RACK-LOS 
ANGELES OFFICE. 

in the Independent field, and one with which the writer happens 
to be very familiar, is that of the Home Telephone Company, of 
Los Angeles, California. The plant is unique in that it is believed 
to be the largest multiple board (present equipment 9.160 linos'). 
in the United States, and that there is but one other board having 
the same ultimate capacity (18,000 lines). This board and the 
entire central office equipment was installed by the Kellogg Switch- 
Board and Supply Company. The building wa s erected specially for 
the purpose, and is used by the telephone company exclusively. Tt 
is a three-story structure, fire-proof throughout. 



684 



AMERICAN TELEPHOXE PRACTICE. 



The basement of the building contains a large room for storage 
besides the usual heating and ventilating apparatus and other equip- 
ment found in a modern building of this size. The first floor is 
devoted entirely to the general offices and reception rooms of the 
company, and nothing has been spared to enhance its architectural 
beauty. 

In Fig. 492 is shown a view of the foyer on the main floor, 
showing the windows leading to the cashier's office on the right 
hand, and the main stairwav on the second floor to the left. In one 




FIG. 497. 



-GENERAL VIEW OF TERMINAL AND POWER ROOM— LOS 
ANGELES OFFICE. 



corner of this room is placed a row of long-distance booths serving 
as a public long-distance station. 

In Fig. 493 is shown the cable vault by means of which all the 
underground cables enter the exchange. This picture was taken 
before all the cables were drawn in. This vault is under the side- 
walk in front of the building, and communicates directly with the 
main cable run leading to the second floor. 

Fig. 494 shows a view of the cable run at a point where the 
cables turn from a horizontal to a vertical direction, after which 
they extend through a narrow cable shaft to the second floor, this 



CENTRAL OFFICE EQUIPMENTS. 



685 



shaft being approximately 20 feet long by 8 inches wide. There 
were at the time this view was taken thirty-two 300-pair cables in 
this run, thus giving a cable capacity of 9,600 lines. 

The floor plan of the terminal room is shown in Fig. 495, and 
from this it will be seen that the main distributing frame and 
relay rack and the intermediate frame are .mounted side by side 
in accordance with the method of construction already referred to. 
The main frame occupies a position alongside of the cable shaft, and 
the line cables, after being terminated in pot-heads, are extended to 




FIG. 498.— POWER PLANT— LOS ANGELES OFFICE. 



the various line terminals of the main frame. The lead-covered 
cables leading up through the shaft are bent around a quarter-round 
grooved beam at the level of the terminal room floor. This supports 
the cables in the shaft, and from this support the cables extend in 
a horizontal direction to the main distributing frame terminals. 

A good idea of the arrangement of the frames may be had from 
Fig. 496, the main frame being on the left, the intermediate frame 
on the right and the relay rack in the middle. 

This view is taken from the front end of the building looking 
toward the rear. 

In Fig. 497, the opposite ends of the distributing and relay 



686 



AMERICAN TELEPHONE PRACTICE. 



frames are shown, this view also showing a general view of the 
power plant and power switch-board. In the center of this view, 
supported by iron posts, is shown a cable run leading from the 
end of the intermediate distributing frame, by means of which a part 
of the intermediate to answering jack cables are taken to the switch- 
board, all answering jack cables, from the ist to the 17th section, 
being fed through the main cable run at the other end of the inter- 
mediate frame while those for sections beyond the 17th are fed 
through this cable run. By this means a saving in cable w T as made 
possible. 

In the rear left hand portion of this view (Fig. 497) is also shown 




FIG. 499.— BATTERY ROOM— LOS ANGELES OFFICE. 



a cable run leading from the power switch-board and fuse-board to 
the floor above and through this run is fed all power and battery 
wires to the switchboard. 

In Fig. 498 is given a general view of the power plant. All wir- 
ing between the machines and the power board is in ducts im- 
bedded in the cement floor. The machines, both charging and 
ringing, are of the Holtzer-Cabot type. 

In Fig. 499 is shown the battery room, this containing two bat- 
teries of ten cells each of chloride accumulators, each cell having 
eleven plates. Sufficient capacity is provided in the tanks for adding 
plates enough to supply the future growth of the switch-board up 
to its ultimate capacity. 




GST 



CENTRAL OFFICE EQUIPMENTS. 



689 



In Fig. 500 is shown a plan of the operating room. The turning 
section at the end of the first section is placed over an opening 
in the floor, through which passes a cable run leading from the 
intermediate distributing frame below. The present arrangement 
of the switch-board is in accordance with ordinary practice, except 
that on account of the narrowness of the room it was necessary 
to make a slightly shorter turn than is usually the case. The turn 
was made on four sections instead of five, there being 36 angles 
between the sections on the turn, instead of 30 , as is usual. 

This switch-board is arranged with a trough plug shelf. The 



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FIG. 502.— LOCKER ROOM-LOS ANGELES OFFICE. 



front of the key shelf is of the ordinary height, thus allowing suf- 
ficient knee-room for the operators, but the key-shelves slope down- 
wardly toward the rear, so that the plug shelves are about 3 inches 
below the level of the front of the key-shelves. In this way the jack 
space is made to extend lower than would be the case were the flat 
key and plug-shelf used, and a greater multiple jack space is thereby 
attained. A general view of the operating room is shown in Fig. 
501, this view having been taken before the work of installation 
was fully finished. 

In Fig. 502 is shown a view of the operators' locker room. 
There is nothing particularly instructive about this view, except 
44 



690 AMERICAN TELEPHONE PRACTICE. 

that it illustrates the modern practice of using lockers composed 
entirely of iron lattice for the operators' clothes and effects. By 
this construction the lockers are made much more hygienic, always 
being properly ventilated. On account of the open construction 
of the lockers, inspection of their contents is also facilitated. 



CHAPTER XXXV. 
AUTOMATIC SWITCH-BOARD SYSTEMS. 

In an early edition of this work this statement is made concerning 
the subject of automatic telephone switch-boards: "It is with the 
idea of following briefly, though not completely, the growth of an 
interesting phase of telephone work, rather than of attempting to 
chronicle any really practical developments, or of giving any hopes 
of its future practicability that this chapter is written." During the 
last few years, and particularly within the years 1903 and 1904, con- 
ditions have greatly changed, and the automatic switch-board, or 
more particularly the so-called automatic exchange, has come into 
such prominence through the efforts of its developers and promoters 
as to make it appear a decidedly important factor at the present 
date, and one that will be of increasing importance in the future. 

The main idea of the automatic system is to dispense with the 
central office operator, switches being so arranged at the central 
office that they will, without the aid of human hands, perform the 
necessary acts of connecting lines for conversation, and afterwards 
disconnecting them at the will of the subscribers. The central 
office switches are governed in their movements by the actions of 
the subscribers or users who desire connections and subsequent dis- 
connections; the subscriber doing his own work, manipulating the 
apparatus before him in such a way as to cause the switches at the 
central office to select, connect with, and afterwards disconnect 
from, the line of the subscriber desired. 

As early as 1879, Messrs. Connelly & McTighe, of Washington, 
D. C, conceived the broad idea of having machines perform the 
entire work of switching lines; and they worked out a set of appa- 
ratus for the purpose that, while somewhat crude in design, never- 
theless embodied the generic principle of modern automatic 
systems. Connelly & McTighe's system consisted essentially of a 
line leading from each subscriber's station to the central office, and 
provided at the sub-station end with a switch whereby it could be 
connected to a make and break impulse-sending device, or to the 
telephone set. With his switch turned to connect his make and 



692 AMERICAN TELEPHONE PRACTICE. 

break device with the line, the subscriber would cause current im- 
pulses to affect the magnets belonging to his line at the central 
office and thereby control the step by step devices which would con- 
tinue his line into connection with any other line in the exchange. 

Connelly & McTighe's patent, No. 222,458, was granted Decem- 
ber 9, 1879, an d is well worth reading by those interested in this first 
of all automatic systems. 

It will be obvious, even from a casual inspection of the figures of 
the Connelly & McTighe patent, that a limit would soon be reached 
in the number of subscribers who could conveniently be handled by 
the machines described; and a little thought will show that the diffi- 
culties caused by the ever-increasing number of subscribers, even 
in the development of the manually operated switch-board, will be 
much sooner reached and apparently harder to overcome when 
dealing with the problem of automatic switching. 

Some years after Connelly & McTighe devised their first system, 
Almon B. Strowger made a number of inventions which were prob- 
ably the first steps toward a practical realization of the ideas ad- 
vanced by Connelly & McTighe. The most important operation 
in all of Strowger's work was that of simplifying the contacts for the 
different line wires, and arranging them on plain or curved surfaces 
so that the selecting arm or point under the control of the stepping 
magnets, operated by the subscriber, would, instead of making a 
continuous forward progress from the beginning to the end of the 
whole series of contacts, be moved relatively to said contacts in one 
direction to select a given series of contacts from among a number 
of such series, and then in a given direction at right angles to the 
first, to select the particular one of the series that might be desired. 
In an early embodiment of this idea by Strowger, a glass cylinder 
was employed, having embedded in it a number of rows of contacts, 
each contact connected to its own line wire and all adapted to be 
swept over by a contact arm on the inside of the cylinder. The 
shaft on which the contact arm was mounted had a vertical move- 
ment within the cylinder, so that the arm might be brought into line 
with any required circular row of terminals. Such movement was 
imparted to the shaft by a magnet operating a pawl, engaging a 
ratchet in such manner as to step the shaft in the direction of its 
length. Beside the longitudinal motion of the shaft, a rotary motion 
was made possible, this being controlled by a pawl, engaging the 
ratchet wheel, the pawl being adapted to receive successive move- 
ments imparted to it by another electromagnet, and thus cause the 



AUTOMATIC SWITCH-BOARD SYSTEMS. 693 

ratchet wheel with its shaft and contact arm to rotate in its bear- 
ings, thus causing the contact arm to pick out any one contact of the 
series opposite which it had been brought by the action of the first 
mentioned pawl. 

This system was of value only in that it contributed an apparently 
important idea to the art, the idea of moving the selecting point first 
in a longitudinal and then in a rotary direction. The defects of 
the system are obvious and great. A very heavy battery, strong 
enough to operate the central office switches over the lines, was re- 
quired at each subscriber's station, and moreover, not less than five 
wires were necessary from each sub-station to the central office. 
Like the Connelly & McTighe patent, this early Strowger patent is 
interesting, if only from an historical standpoint. It was granted 
March 10, 1891, its number being 447,918. 

In 1894 a system was produced by Messrs. Keith, Lundquist, J. 
and C. J. Ericson, which was tried in commercial use, and although 
abandoned, served as an impetus to further effort on the part of the 
same gentlemen. In this a series of wires were tightly stretched 
on a rack, parallel to each other, there being one such wire for each 
line going into the office! Each telephone line also had a mechan- 
ism consisting of a shaft carrying contact arms or "wipers, " the 
shaft being adapted to be moved in a longitudinal direction, and 
also to be given a rotary movement in a manner not unlike that 
employed by Strowger in his work. A common battery was used 
at the central office to supply the energy needed for the switch- 
moving magnets, current from this battery flowing over the lines to 
the subscribers' stations in response to the closure of the switches 
at these stations. The number of wires was reduced from five to 
two, the rotary movement of the shaft being imparted by a magnet 
adapted to respond to currents in one of the line wires with ground 
return; and the vertical movement being similarly imparted by a 
magnet responding to currents in the other wire with ground return. 

The parallel wires at the central office were divided into groups of 
ten, and each shaft had as many arms as there were groups of ten 
wires, the arms being differently set on the shaft with respect to 
their angular placement, so that but one arm at a time could engage 
any of the wires. 

Four keys were placed on each subscriber's instrument marked 
"H" (hundreds), "T" (tens), "U" (units), and "R" (release). If a 
subscriber desired to call No. 143, he would press the hundreds key 
once, the tens key four times, and the units key three times, which 



694 AMERICAN TELEPHONE PRACTICE. 

would bring one of the contact arms on the shaft into engagement 
with one of the parallel wires forming the terminals of line No. 143. 
This system was apparently adapted to serve one hundred lines only, 
as a maximum. 

In the present Strowger system, as it is now manufactured by the 
Automatic Electric Company, of Chicago, the parallel wire method 
of switching has been abandoned; but the switches having both 
longitudinal and rotary motions have been retained, this idea form- 
ing the basis of the entire system. The idea of trying to make the 
switch contacts of any line play over a large number of the con- 
tacts, in order that the selecting contacts of any line may be brought 
directly into contact with the stationary contacts of the line wanted, 
has also been abandoned, and in its stead a system of trunking has 
been brought into play. By this means what are called primary 
switches, or first selectors, and secondary switches, termed "second 
selectors" and "connectors," are used, and through their action the 
connection between any two lines is built up, a section at a time, 
rather than by a single connecting link, as was the idea of all the 
early inventors. 

In order to understand the system as it is at present installed in 
a number of large offices, it may be said that each line terminates in 
a switch known as a first selector, the circuit of the line passing 
through certain magnets of this switch and terminating under cer- 
tain conditions in the selecting contact points or wipers of this 
switch. Mounted upon the framework of the switch and within 
range of the wipers, are what are called "banks of contacts," there 
being 100 sets of the contacts for each bank, the contacts being 
arranged in ten rows of ten each. The construction of the switch 
is such that for each step in the longitudinal direction of the shaft, 
which is vertical, the wipers in which the line wires terminate, are 
brought opposite a different horizontal row of bank contacts, and 
after the shaft is brought to the required height, that is, opposite the 
required row of bank contacts, it may be given by another magnet 
a rotary movement which causes the wipers to engage any particular 
contact in that row. „ 

Assuming that the exchange is adapted to accommodate 10,000 
lines, or more properly, is adapted to accommodate lines num- 
bered up to 9999, each row, or "level," as it is sometimes termed, in 
the bank contacts of the first selector switch, will represent trunk 
lines leading to a group of second selectors for a particular thou- 
sand. That is, the bank contacts in the first row or level of the first 



AUTOMATIC SWITCH-BOARD SYSTEMS. 695 

selector, will be connected by trunks to second selectors adapted to 
extend connections to subscribers having numbers beginning with 
iooo. The second row of bank contacts will be connected by trunks 
to second selectors adapted to continue the connections to lines 
bearing numbers in the two thousands, and so on. In order not to 
unduly multiply the number of trunks thus leading from the first to 
the second selectors, the first selectors are divided into groups, in 
later exchanges of a thousand each, and all of the corresponding 
bank contacts of the first selectors in this group are multipled to- 
gether after the manner of multipling spring jacks in a multiple 
switch-board. For the present understanding it is sufficient to say, 
therefore, that the first contact in the first row of all the first selec- 
tors in a given thousand, are connected together, and to a trunk line, 
this trunk line terminating in a second selector in practically the 
same manner as does a subscriber's line in a first selector. As there 
are contacts for ioo trunk lines on each first selector, it follows, 
therefore,. that there are, for each thousand first selectors, ioo trunk 
lines leading to ioo second selectors. These ioo second selectors 
belonging to any group of iooo first selectors, are divided into 10 
groups of 10, it being remembered that the trunk lines leading from 
the bottom row of contacts on the first selectors are ten in number, 
and leading to ten second selectors in the first thousand. There 
are, therefore, for each thousand first selectors, 10 groups of 10 
second selectors, each group .of second selectors corresponding to 
a given row or level of contacts on the first selectors, and each rep- 
resenting a possible connection from any one of its group of first 
selectors to any wanted line in a particular thousand. 

The action of the first selector with respect to the trunks of the 
second selectors is this: A series of impulses sent over one side of 
the subscriber's line and ground causes the shaft of the first selector 
to move up a step at a time until it gets opposite the row of con- 
tacts containing trunk terminals of the proper thousand in which 
the called-for subscriber's number exists. The switch arm is then 
given a rotary motion until it picks out a trunk line leading to a 
certain selector in that thousand that is not busy. It is obvious 
since the bank contacts of the first selectors are multipled. that un- 
less special arrangements are made to guard against it, the wipers 
on different first selectors might stop on the same trunk in the 
same manner as, without a busy test, two operators in a manual 
system might plug into different multiple jacks of the same line at 
the same time. This difficulty is met by so arranging the circuits 




FIG. 503.— SELECTOR SWITCH, FROXT VIEW 
696 




FIG. 504.— SELECTOR SWITCH, SIDE VIEW 
697 



698 AMERICAN TELEPHONE PRACTICE. 

that the rotary movement of the wipers will continue until the 
wipers engage a trunk line that is not in use. 

By this means, that is, by the first series of impulses sent over 
the subscriber's line in making a call, the line of the subscriber is 
extended through the first selector to a second selector belonging 
in that group of a thousand lines in which the called-for subscriber's 
line terminates. 

The next series of impulses that is sent over the subscriber's line 
operates in a like manner on the second selector, to cause it to pick 
out a trunk line extending to the connector belonging to the particu- 
lar group of ioo lines in which the called-for subscriber's line exists. 
In order to accomplish this, the bank contacts on each group of 10 
second selectors are multipled together, and these extend by trunk 
lines to the wipers of the connectors, the relation between the con- 
nectors and the second selectors being the same in this respect as 
that between the second selectors and the first selectors. The sec- 
ond series of impulses sent by the subscriber, therefore, moves the 
shaft of the second selector until it is opposite the row of contacts 
in which terminate the ten trunks leading to the connectors of the 
particular hundred desired, and after that a rotary impulse is im- 
parted to the shaft of the second selector, which causes it to revolve 
until an idle trunk contact is engaged; whereupon the connection 
is continued from the subscriber's line through the first and second 
selectors to the connector. The connector is a switch that is identi- 
fied with a particular group of ioo lines. There may be, and usually 
are, 10 connectors for each ioo lines, and the bank contacts of these 
connectors are multipled together in the same manner as are the 
bank contacts on the first and second selectors. Instead, however, 
of these multiple bank contacts forming the terminals of trunk lines, 
they are connected directly with the lines of the subscribers in the 
group of ioo to which the particular ten connectors are assigned. 
There being ioo sets of bank contacts on each connector, all these 
being multipled together on the entire group of connectors belong- 
ing to that hundred, it follows that each line has one contact in the 
bank of each of the ten connectors belonging to that group. The 
second series of impulses sent by the subscriber serves, as has been 
said, to continue his line to an idle connector within the ioo to 
which the line of the called-for subscriber belongs. 

The third series of impulses sent by the subscriber will merely 
move the shaft of the connector up until it is opposite the row or level 
of contacts in which the line of the called-for subscriber belongs. 




FIG. 505.— SELECTOR SWITCH. REAR VIEW 
G99 



700 AMERICAN TELEPHONE PRACTICE. 

The remaining selection, therefore, is only one of ten, and the fourth 
series of impulses merely gives the wipers of the connector a rotary 
motion of as many steps as are necessary to bring the wipers into 
engagement with the bank contact on the connector belonging to 
the line of the subscriber called for. 

The calling subscriber then, by means of pressing the ringing 
button on his telephone, completes such a circuit condition as to 
cause the operation of a relay on the connector, which in its action 
performs exactly the same functions as does the ringing key in a 
manual exchange. That is, it cuts off the line behind it and estab- 
lishes connection between the terminals of the calling generator 
and the line of the subscriber called. If the called subscriber's line 
were already in use, or busy, the connector would not have con- 
nected with it, but would have dropped back to its normal position. 
Under this circumstance, when the calling subscriber presses his 
ringing button, immediately afterwards he receives the well-known 
buzz in his receiver, indicating that the line is busy. 

Having now considered the general method by which the con- 
nections are established in the automatic exchange, consideration 
may be given to the details of the switching apparatus, and to the 
various circuit arrangements by which they are connected and en- 
abled to perform one of the most remarkable series of actions 
and interactions that has ever been attained by mechanical struc- 
tures. A good idea of the structure of the first selectors may be 
obtained from an examination of Figs. 503, 504 and 505. Of these, 
503 shows a front view of the switch, the shaft being clearly shown, 
and on it near its top, the series of notches in which the pawl en- 
gages to actuate it in its vertical movements. At the bottom of this 
figure the bank contacts are shown, these being arranged in three 
sections. The two lower sections, consisting of five double rows 
each, carry the talking contacts of this switch. There are 100 pairs 
of these contacts, the lower five rows of these being adapted to be 
engaged by the lower pair of wiper contacts on the shaft, and the 
upper five rows — that is, those in the bank of the section in the 
middle — being adapted to be engaged by the second pair of wiper 
contacts on the shaft. These 100 pairs of bank contacts might, so 
far as their switching function is concerned, be arranged in a single 
section; but in order that the vertical steps by which the shaft moves 
might not be too long, or the insulation between the rows of con- 
tacts too thin, they are divided into two sections, as shown. The 
lower pair of wipers is brought into engagement with the lower 



AUTOMATIC SWITCH-BOARD SYSTEMS. 701 

row of the lower section by the first vertical movement of the shaft. 
The second vertical movement of the shaft brings the second pair 
of wipers into engagement with the lower row of the upper section 
of the line contacts, the lower pair of wipers then occupying a space 
half way between the first and second rows of the lower section. 
The third vertical movement brings the lower wipers into engage- 
ment with the second row in the lower section, and the fourth verti- 
cal movement the second pair of wipers into engagement with the 
second row of its section. The two lower pairs of wipers on the 
shaft are connected in multiple, and thus the same result electrically 
is secured as if the whole ioo pairs of bank contacts were engaged 
by a single pair of wipers rather than by two pairs. According to 
this arrangement, the five rows of bank contacts in the lower sec- 
tion form the levels for the first, third, fifth, seventh and ninth thou- 





FIG. 506.— BANK CONTACTS. 

sand. The rows in the second section form the levels of the second, 
fourth, sixth, eighth and naught thousand. 

So far, no mention has been made of the upper section of bank 
contacts, and this, it may be stated, is called the "private" or "busy" 
bank. It consists of ten rows of ten contacts, each adapted to be 
engaged by the wiper carried by the same shaft as that on which the 
wipers for completing the talking circuit are carried, and moving 
in unison therewith. The function of these private or busy con- 
tacts will be pointed out when the circuits are considered. 

A side view of the switch shown in Fig. 504 gives some idea of 
the details of what is termed the "side switch." This is composed 
of five springs shown just below the lower magnet in this cut. Fig. 
505 shows the rear view of the switch, the terminals of the bank 
contacts being clearly shown denuded of multiple wiring. 



702 



AMERICAN TELEPHONE PRACTICE. 



In Fig. 506 is shown the method of building up the banks of 
contacts, a complete bank being shown in one view of this figure, 
and a single row of multiple contacts in the other. The con- 
tacts in these banks are clamped between layers of fibre, and bolted 
together as shown, in a way that is at once ingenious, reliable in 
point of permanency, accuracy and insulation, and, moreover, not 
unduly expensive to construct. 

The telephone or subscribers' equipment is shown in Fig. 507. 




FIG. 507.— AUTOMATIC TELEPHONE. 

The only part of this that is of special interest is the impulse trans- 
mitting device, of which a front view is shown in Fig. 508, a rear 
view in Fig. 509, and a side view in Fig. 510. The dial, shown 
most clearly in Fig. 508, is mounted on the front of the telephone 
box, as shown in Fig. 507. It has in it a series of ten holes, into 
which the forefinger of the subscriber is adapted to fit. In order 
to send the proper impulses to the central office to secure a con- 
nection with a line of any desired number, the finger of the 



AUTOMATIC SWITCH-BOARD SYSTEMS. 



703 



subscriber desiring the connection is first placed in the hole corre- 
sponding to the first digit in that number, and the dial is pulled 




JZJm 




Nfc_*_# 



FIG. 508.— TRANSMITTING DEVICE, FRONT VIEW. 

around, as shown in Figs 507, until the finger strikes a stop mounted 
on the front of the telephone box, as is clearly shown. The dial is 




j^y 



VL3 

FIG. 509.— TRANSMITTING DEVICE, REAR VIEW. 



then released and returned to its normal position by a retractile 
spring, and in so doing the notched wheel within the box causes 



704 AMERICAN TELEPHONE PRACTICE. 

a pair of springs to make a series of contacts, serving to im- 
part a vertical movement to the first selector of the subscriber 
calling. The operation is then repeated for the second, third and 
fourth digits in the number, the second movement of the dial impart- 
ing to the second selector the required vertical motion, the third 
doing the same for the connector, and the fourth serving to rotate 
the arm of the connector into engagement with the line wanted, as 
already described. 

The circuit and some of the mechanisms of the subscriber's in- 
strument diagrammatically arranged are shown in Fig. 511. This 
is typical of the latest desk-stand telephone of the Automatic Elec- 




FIG. 510.— TRANSMITTING DEVICE, SIDE VIEW. 

trie Company, this being somewhat more simple than is the wall set, 
although its electrical functions are identical therewith. 

The two sides of the line which, in manual practice, are designated 
by such terms as "tip" and "sleeve," or "tip" and "ring," are, in 
automatic practice, designated as "vertical" and "rotary." These 
names are given the limbs of the line because over the vertical side 
of the line are sent those impulses which impart to the switch its ver- 
tical motion, and over the rotary side of the line single impulses 
are sent, as a result of which the various switches are started in their 
rotary movements. The letters, "V" and "R," will be used through- 
out this discussion as designating the vertical and rotary sides of 
the line, respectively, or such extensions thereof as may be brought 



AUTOMATIC SWITCH-BOARD SYSTEMS. 



705 



about at the central office as a result of the movements of the 
switches. 

The two limbs, V and R, are shown in Fig. 511, and in all posi- 
tions of the switching mechanism these lead directly to the two 




FIG. 511.-CIRCUITS OF SUBSCRIBER'S INSTRUMENT. 



impulse-sending springs, 1 and 2. These two springs are so moved 
by the dial of the impulse transmitter, the action of which has 
already been explained, that upon the return movement of the dial 
45 



706 AMERICAN TELEPHONE PRACTICE. 

the spring, i, makes as many contacts with the ground spring, 3, as 
will correspond to the digit in the number of the called subscriber 
which is at that time being selected. Immediately after this series 
of impulses, which result in the flow of current over the vertical 
side of the line, a single contact is made at the end of the return 
movement of the dial by forcing the spring, 2, into engagement with 
the ground spring, 3, this resulting in a single impulse over the 
rotary side of the line. To make this clear, if the number of the 
called subscriber is 5692 the subscriber would first, after removing 
his receiver from the hook, put his finger in the hole marked 5 (as 
shown in Fig. 507) and pull the dial down to the stop. Upon re- 
leasing the dial five impulses of current would flow over the vertical 
side of the line, followed by one over the rotary side. He would 
then repeat the motion, using the holes 6, 9 and 2, respectively, in 
the order mentioned, the corresponding number of impulses being 
sent over the vertical side of the line upon the return of the dial in 
each case, each series of vertical impulses being followed by a single 
rotary impulse. 

This succession of vertical and rotary impulses must be firmly 
rooted in one's mind before a proper understanding may be had of 
the automatic system, as at present developed. It must be borne 
in mind that no series of vertical impulses may be sent without being 
immediately followed by one rotary impulse, and one only. 

Returning now to consideration of Fig. 511, it will be noted that 
there is a pair of contacts, 4 and 5, controlling the circuit from the 
spring, 3, to ground. This pair of contacts is normally open by 
virtue of spring, 5, being held out of engagement with the contact, 
4, by the upwardly extending arm, 6, on the hook. When the hook 
is raised the spring, 5, is prevented from making engagement with 
its contact, 4, by the presence of the dog, 7, which lies within the 
path of an insulated bushing on the spring, 5. This dog, 7, is 
moved out of engagement with this bushing as soon as the dial is 
moved off its normal position. It is, therefore, not until the hook 
has been raised and the dial started that any ground connection is 
secured at the subscriber's station. It may be also said in con- 
nection with the dog, 7, that it serves as a lock for the dial, prevent- 
ing any movement of the dial until the hook has been relieved from 
the weight of the receiver. 

When the hook is down the circuit through the magneto bell of 
the station is completed from the rotary side of the line through the 
bell magnets to the spring, 10, thence through the contact, 13, on 



AUTOMATIC SWITCH-BOARD SYSTEMS. 707 

the hook and the wire, 14, to the vertical side of the line. This 
circuit is broken in an obvious manner when the hook is raised. 
The primary circuit containing the transmitter, battery and primary 
winding of the induction coil, is closed in the ordinary manner 
when the hook is raised at the contacts, 11 and 12. Trie secondary 
circuit, including the receiver and secondary winding of the induc- 
tion coil, is closed when the hook is up by the engagement of the 
springs, 8 and 9, but this circuit is opened by the movement of the 
dog, 15, which is mounted on the shaft of the dial and turns there- 
with; the arrangement being such that as long as the dial is "oft" 
normal" the arm of the dog, 15, will not press the spring, 8, into 
engagement with the spring, 9. This is to secure the disconnection 
of the two sides of the line during the transmission of the impulses 
as described, and this arrangement is necessary to prevent impulses 
from being sent over both sides of the line at once when the springs, 
1 and 2, are in action. 

The circuit over which voice currents pass when the hook is up 
and the dial not in motion is traced from the rotary side of the line 
through the secondary of the induction coil and the receiver to the 
spring, 16, of the ringing key, thence through the spring, 17, of this 
key to the springs, 9 and 8, of the hook, the latter spring being 
connected to the vertical side of the line. 

When the ringing key is pressed the spring, 17, engages the 
spring, 18, thus breaking the talking circuit and grounding the 
vertical side of the line. 

The springs, 19, 20 and 21, are so disposed with relation to the 
hook lever that they will not be forced into engagement by the up- 
ward motion of the hook, but will be brought into engagement 
momentarily while the hook is being depressed. These springs are 
normally out of engagement with each other in either position of 
the hook, their only time of engagement being while the hook is 
returning to its normal position. When they are caused to thus 
engage each other it is obvious that both the vertical and rotary 
sides of the line will be grounded, if the contact at 4 and 5 has been 
previously closed by the movement of the dial. This condition is 
that which causes the restoration of all the switches at the central 
office, which are connected with this line at the time, to their normal 
position. 

If the salient points in the operation of the subscriber's mechanism 
■ be borne firmly in mind an understanding of the very complex se- 
quence of events at the central office will be made easier. A re- 



708 AMERICAN TELEPHONE PRACTICE. 

capitulation of these points is as follows : Each movement of the 
dial causes a series of impulses to -flow over the vertical wire, fol- 
lowed in each case by a single impulse over the rotary; each pressure 
on the ringing key opens the talking circuit at the subscriber's sta- 
tion and grounds the vertical side of the line; each depression of the 
hook after the dial has been moved causes both the vertical and 
rotary sides of the line to be grounded for clearing out purposes. 

Passing now to the central office, the circuits of the first selector 
are shown in some detail in Fig. 512. In this the two wires, V and 
R, leading from the subscriber's station just described, and shown 
in Fig. 511, may be seen entering at the left. These terminate in 
the first and second levers of the side switch, these levers being 
numbered for the purpose of easy description from 1 to 5 as shown. 
It must be remembered that all of the levers of the side switch move 
in unison; that their normal position is on the left-hand contacts as 
shown in the various figures; and that their position during con- 
versation is that in which they engage the right-hand contacts, this 
being the position in which they are shown in Fig. 512. It must 
further be remembered that the side switch levers are controlled in 
their movements by the action of the private magnet, P M. This 
method of control is peculiar: a spring tends to hold the side switch 
in its right-hand position; it is normally held, however, in its left- 
hand position by an escapement mechanism on a lever controlled 
by the private magnet. When the private magnet is energized and 
de-energized once, the escapement allows the side-switch arms to 
move one step, thus engaging their middle contacts. A subsequent 
attraction and release of the private magnet armature will result 
in the release of the side-switch levers one more step, thus bringing 
them into engagement with their right-hand contacts. The arrange- 
ment by which this action of the side switch is brought about is 
partially shown pictorially in the diagram in connection with the 
private magnet, P. M. The forwardly projecting arm on the arma- 
ture of this magnet is notched as shown, and these notches serve to 
retain an arm, 22, of triangular cross section, rigidly attached to 
the side-switch levers. When the armature of the private magnet 
is attracted the arm, 22, tends to move toward the left, but is held 
by a notch in the spring lying just under this arm, and is only re- 
leased therefrom when the private magnet armature is released. 
The arm, 22, is then engaged by the second notch on the private 
magnet armature, the side switch then being in its middle position. 
The subsequent attraction and release of the private magnet arma- 



TO MULTIPLE BAMK 
CONTACTS OF C0HNECT0R3 
SERVING THE QROOP OF 100 
TO WHICH THIS SWITCH BEU0NQ3. 




710 AMERICAN TELEPHONE PRACTICE. 

ture will result in the moving of the side switch to its third position. 

This action of the side switch, and its control by the private mag- 
net, is one of the salient features in the operation, not only of the 
first selectors, but of the second selectors and connectors, and must 
be clearly kept in mind in following the actions of all these switches. 

The two sides of the line pass from the side-switch levers to the 
vertical and rotary relay coils marked V R and R R, respectively, 
and from these the circuits of the two sides of the line are continued 
through the springs, 23, 24 and 25, of the cut-off relay, C 0, to the 
live side of the battery, B. These relays are thus placed in such 
relation as to be actuated over their respective line wires when the 
grounding of these wires occurs in response to the movements of 
the dial and of the release springs at the subscriber's station. The 
function of the cut-off relay, C 0, is to break the circuit under cer- 
tain conditions through these two relay magnets, this function being 
pointed out in its proper place. R M and V M are, respectively, the 
rotary and vertical magnets which impart to the shaft of the switch 
its rotary and vertical movements. It will be noted that the vertical 
magnet can have its circuit completed only when the side switch is 
in its first position, while the rotary magnet can have its circuit 
completed only when the side switch is in its second position. 

The disconnect or release magnet is shown at D M, and this is so 
connected that its circuit will be completed only when both the ver- 
tical and rotary sides of the line have been grounded at the sub- 
scriber's station, the conditions under which this occurs being more 
readily understood as this description progresses. 

At the top of the diagram in Fig. 512 is shown what is called an 
off-normal switch and an off-normal lamp. The contacts of this 
switch are held apart by a collar on the shaft of the selector switch, 
the arrangement being such that as long as the selector shaft is in 
its normal position the off-normal switch will be open, while as 
soon as the shaft is moved upward one step or any number of steps 
the off-normal switch will be closed and a lamp common to a group 
of switches be lighted, if the small hand switch, 26, is closed. The 
object of this is to afford facility for the detection of any switch or 
switches in a group which may be, through some fault in the work- 
ing of the system, left in other than normal position. At / is 
shown a constantly driven interrupter, the purpose of which is to 
afford to the rotary magnet periodically recurring current impulses 
from the battery, B, during the time while the side switch is in its 
second position. 



AUTOMATIC SWITCH-BOARD SYSTEMS. 



Ill 



Before passing to the operation of the selector switches under the 
various circuit conditions, attention should be called to the fact that 



FIG. Ck SCHEMATIC CIRCUIT BEFORE ee<3lHMlM<3.C*U-. 
SUBSTATION FIRST SELECTOR 
. CALLING. CALLING, 



rmarr selector substatioh 

TO £E CALLEp. TO EiE CNJUEp. 





FIQ. b. SCHEMATIC TALKING CIRCUIT AFTER rULUNG FIRST PIGIT. 



SUBSTATION I W. SELECTOR ZHPSELCCTOR. 
CALUNG. CALLING TRUNKING. 



\«r. selector substation 

TO BE CALLEP. TO.BE CAULEp. 





FIQ.C SCHEMATIC TALKING CIRCUIT AFTER PULLING 3ECONP PlGlT. 

I «* SELECTOR SUBSTATION 

TO E»E CALLEP TO BE CALLER 

v*R> A , ^BRR 



SUBSTATION I ST. SELECTOR- Z*?SELECTOR CONNECTOR. 

CALLING CALLING TRUNKIN^ TRU NKIN G 

O T 




"~T7" T > 



R T R* T R* 



=> $BSR 




FIG- A. SCHEMATIC TALKING CIRCUIT OF A COMPLETED CONNECTION. 
SUBSTATION ^SELECTOR - Z*£ SELECTOR CONNECTOR. IV SELECTOR SUBSTATION 



CA.LLING CALLING TRUNKING 
VL 




vr: 



1 



CALLEP 

_VL 



I rrJ RrJ '. K P»R 

L 1 ^JJiJ 



.VR 



rv; 



FIG. 613.— SCHEMATIC REPRESENTATION OF THE VARIOUS STEPS IN 

A CONNECTION. 



the two left-hand contacts of the side-switch lovers, 1 and 2, arc con- 
nected by wires designated respectively as "rotary normal" and 



712 AMERICAN TELEPHONE PRACTICE. 

"vertical normal," to the multiple bank contacts of the group of con- 
nector switches, serving the group of subscribers to which this 
particular first selector belongs. The lever, 3, of the side switch is 
permanently connected by the "private normal" wire to the private 
bank contacts on the same connector switches. These normal wires 
form the path for the incoming calls to the line of this first selector, 
and it will be noticed that the contacts between the vertical and 
rotary line wires, V and R, and the corresponding vertical and rotary 
normal wires is severed as soon as the side switch of this first 
selector is moved out of its normal position. 

At the lower portion of Fig. 512 are shown the wipers carried by 
the shaft and also the corresponding rows of private, vertical and 
rotary bank contacts over which these wipers are adapted to move 
when the shaft has been stepped to a certain vertical position. The 
bank contacts shown on this circuit are those of one level only of 
the first selector, and therefore one-tenth of all the bank contacts 
on this selector. The first selectors are divided into groups of one 
hundred and the bank contacts are multipled throughout this group, 
as has already been described. In addition to these multiple con- 
nections the vertical and rotary bank contacts are also connected 
by means of trunk wires to second selectors, these second selectors 
being grouped with respect to the groups of thousands they are to 
serve. The vertical and rotary bank contacts shown in this figure 
are each connected therefore to the second selectors serving some 
particular group of a thousand lines. The ten pairs of contacts in 
each of the nine other levels on the first selector would terminate in 
other second selectors serving respectively other groups of 
thousands. 

Coming now to the actual description of the workings of the 
various parts of the first selector, reference will be made to the vari- 
ous diagrams in Fig. 513, in which the successive steps in building 
up a connection between a calling subscriber's station at the left, 
and a called subscriber's station at the right, are illustrated in a 
schematic way. At the top of this figure is shown the condition of 
the circuit of the called and calling lines before the beginning of a 
call. This condition with respect to the first selector of the calling 
line is shown more in detail in Fig. 514, some of those parts shown 
in Fig. 512 not operative at this time being omitted. 

Referring particularly to Fig. 514, the two sides of the line, 
V and R, are shown, continued respectively to the vertical and 
rotary normals, V N and R N, thus being in a receptive condition for 



AUTOMATIC SWITCH-BOARD SYSTEMS. 



713 



incoming calls from some connector. This is the telephone's condi- 
tion of idleness, the side-switch levers I, 2, 3, 4 and 5 of the first 
selector being at the left. The line wires, V and R, are continued 
to the live pole of the battery, B, through the relays, V R and R R, 
respectively. 

It is evident that the first series of vertical impulses sent by the 
subscriber corresponding to the thousands digit in the number of 
the desired subscriber, will cause a corresponding number of attrac- 
tions of the armature of the vertical relay. This will cause a cor- 
responding number of current impulses to traverse the vertical 



SuaSTATlOM 
CALLING 



ft 







-<—e?=* J ^_ 



FIG. 514.— FIRST SELECTOR— NORMAL. 



magnet, V M, this circuit being traced from battery, B, through the 
magnet, V M, to the left-hand contact point of the fourth side-switch 
lever, thence to the back contact of the private magnet to ground 
through the contacts of the vertical relay. The vertical magnet will 
therefore step the shaft up a corresponding number of points until 
the wipers, P W, V W and R W, are opposite their respective rows 
of contacts corresponding to the trunk lines leading to the proper 
group of second selectors. Immediately afterwards, the rotary re- 
lay, R R, will receive one impulse due to the grounding of the 
spring, 2 (Fig. 511), and as a result of the attraction of its armature 
the private magnet will operate to attract and release its armature 
once. The contacts carried by the private magnet will not in this 



714 



AMERICAN TELEPHONE PRACTICE. 



case effect any circuit changes, since the circuit from its armature is 
open at the vertical relay. The movement of the private magnet 
armature will, however, through its escapement, allow the side 
switch to move to its middle point. 

The circuit is now that of Fig. 515, and from this it will be seen 
that the circuit of the rotary magnet is completed through the fifth 
side-switch lever, the battery, B, and the interrupter, /. The rotary 
magnet thus receives a number of impulses in rapid succession, and 



<^—l 



SUBSTATION 
CALLINQ 




FIG. 515.— FIRST SELECTOR DURING TIME OF ACTION. 



as a result of each one the shaft and its wipers are stepped around 
over the horizontal row of trunk line contacts. 

In view of the fact that the wiper contacts connected with the 
trunk lines are multipled to other switches, it is evident that some 
of the trunk lines, represented by the contacts over which the rotary 
magnet is now causing the wipers to sweep, may be busy at other 
switches. It is the function of the rotary magnet, therefore, to 
move the wipers until they engage the contacts of the first idle trunk, 
and for the purpose of illustration let it be assumed that the first 
two trunk lines of the selected horizontal row are busy at this time. 
When a trunk is busy its private bank contacts are grounded at the 
switch at which the trunk is made busy in a manner that will be 



AUTOMATIC SWITCH-BOARD SYSTEMS. 715 

explained. The first two private wiper contacts of the switch that 
is being rotated are therefore in Fig. 515 shown as grounded. The 
private wiper, P W, when it engages the first private contact, will 
complete a circuit from the live side of the battery, B, through the 
private magnet, side-switch lever, 3, to ground through the private 
wiper. This will serve to hold attracted the armature of the private 
magnet, while another impulse is given the rotary magnet through 
the interrupter. The rotary magnet is thus again attracted to move 
the private wiper and other wiper contacts into engagement with 
the second contacts, where another ground is found by the private 
wiper, resulting in the private magnet being still held attracted and 
another impulse being given the rotary magnet. The private wiper 
now finds in contact 3 no ground, and the private magnet is there- 
fore released, which results in the movement of the side switch to 
its third contact point which primarily accomplishes the result of 
cutting off the interrupted current through the rotary magnet and 
stopping the wipers on the third contact. It will be seen that the 
rotation of the shaft depends on the continued action of the rotary 
magnet, and this depends on the side switch being in its second 
position. This in turn depends on the private magnet not being re- 
leased while the private wiper is sweeping over busy trunk contacts, 
for as soon as it is released it causes the movement of the side switch 
to its third position. In order that the private magnet armature 
may not fall back while the private wiper is passing, in its rotary 
movement, to the first private multiple contact or from one private 
contact to another, a certain mechanical relation exists between the 
armature of the rotary magnet and that of the private magnet. The 
rotary magnet armature carries a finger which projects in front of 
the private magnet armature and results in the private magnet arma- 
ture being held up, as though attracted, during the attraction of the 
rotary magnet armature. It may, therefore, be stated that while 
the wipers are sweeping over a series of busy trunk line contacts the 
private magnet armature is held attracted electromagnetically 
while the private wiper is on a busy trunk line contact, and during 
the interval while the wiper is passing to the first contact, or from 
one to another, the private magnet armature is held up by the 
rotary magnet armature, which is then in operation. This inter- 
action between the private magnet armature forms an important 
link in the chain of actions in all the selector switches. 

The movement of the side switch into its third position brings 
about the state of affairs shown in Fig. m6. Beside cutting off the 



16 



AMERICAN TELEPHONE PRACTICE. 



circuit through the rotary magnet, the movement of the side switch 
into its third position causes other important circuit changes which 
are indicated in Fig. 516. The first of these is the grounding of the 
private wiper at the right-hand contact of side-switch lever 3. Since 
this private wiper now engages the private contact of the trunk line 
to which the vertical and rotary wipers have been connected, it fol- 
lows that all of the private contacts of that trunk on other switches 
are grounded, and this trunk line will therefore be held busy to 
other incoming calls in the same group of subscribers. This ex- 
plains how the ground connections are put on busy trunks, as was 
referred to in conection with Fig. 515. The movement of the side 
switch into its third position also connects the vertical and rotary 
sides of the line with the vertical and rotary wipers, V W and R W, 




FIG. 516.— FIRST AND SECOND SELECTORS AFTER PULLING FIRST DIGIT. 



and the connection is therefore continued from the line to the trunk 
line formed by the two wires, V and R', leading to a second selector. 

The first step in securing the desired connection is now accom- 
plished, and the conditions with respect to the talking circuit are 
those shown in the second schematic diagram of Fig. 513. The 
bridge formed by the vertical and rotary relay magnets, V R and 
R R, of the first selector still exists, while another bridge has been 
added at the second selector in the coils, V R and R' R. The selec- 
tion of the thousands group has been performed and the line of the 
calling subscriber has been brought one step closer to the terminals 
of the called subscriber. 

The action of the second selector is identical with that of the first 
selector; a horizontal row of trunk line contacts being selected by 



AUTOMATIC SWITCH-BOARD SYSTEMS. 717 

the vertical magnet of the second selector in accordance with the 
number of impulses sent over the vertical side of the line in response 
to the second movement of the subscriber's dial. The impulse over 
the rotary side of the line, immediately following, operates the rotary 
relay, which in turn gives the private magnet one impulse, and this 
in turn releases the side switch of the second selector which moves 
to its second position. This brings into play the rotary magnet 
which, as long as the private magnet armature is held attracted by 
virtue of the private wiper engaging busy contacts, continues to 
operate, until the private wiper strikes a non-busy and therefore 
non-grounded contact, which moves the side switch to its third 
position by virtue of the release of the private magnet armature. 
The movement of the side switch to its third position cuts off the 
interrupted current from the rotary magnet, grounds the private 
wiper and continues the trunk line wires, V and R' ', to the wipers, 
V'W and R'W, respectively, and therefore to the trunk line leading 
to the proper connector. 

As a result of the second movement, therefore, the circuit of the 
calling subscriber's line is continued to a connector serving the 
group of one hundred subscribers in which the called-for sub- 
scriber's line exists. The connection thus established is shown in 
Fig. 517, and the schematic diagram of the talking circuit is shown 
in the third diagram of Fig. 513. The calling subscriber's line is 
now extended, as shown in this latter figure, by the addition of the 
trunk line wires, V 2 and R 2 , and a third bridge is added by the verti- 
cal and rotary magnets V 2 R and R 2 R of the connector switch. It 
now only remains for this connector switch to pick out by its vertical 
movement the row of ten contacts in which the called subscriber's 
line exists and then by its rotary movement to pick out the terminal 
of that line from among this ten. 

Upon pulling the third digit of the number being called the series 
of impulses sent over the vertical line will operate the vertical relay, 
V 2 R, of the connector switch as in the case of the first and second 
selectors. This, as before, will cause the vertical magnet, V 2 M, to 
step the shaft up a corresponding number of steps, the side switch 
of the connector being at the time at the left. The wipers of the 
connector are thus moved to a position opposite that row of mul- 
tiple contact points on the connector bank, which form the terminals 
of the ten subscribers' lines containing the desired subscriber. 

The rotary impulse following the third movement of the dial 
energizes the rotary relay, R'~ R, which, as before, energizes the pri- 



718 



AMERICAN TELEPHONE PRACTICE. 



vate magnet, P 2 M, this in turn releasing the connector side switch to 
its middle point. This action in the first and second selectors, it will 
be remembered, served to start the rotary magnet in its work of 
stepping the shaft around in its rotary movement. In the case of 
the connector this is not true, but instead, the lever, 4, of the side 
switch merely places the rotary magnet, R 2 M, under the control of 
the vertical relay in place of the vertical magnet. This action is 
clearly shown in the diagram of Fig. 517, where it will be seen 
that with the side switch in its first position the vertical magnet, 
V* M, is in the local circuit closed by the vertical relay, V 2 R. When 
the side switch shifts to its middle position the rotary magnet, 
R 2 My is placed in the same relation with respect to the vertical relay 




CONNECTOR TR.UNKIN5 




FIG. 517.— FIRST AND SECOND SELECTORS AND CONNECTOR AFTER 
PULLING SECOND DIGIT. 



as was the vertical magnet before. As a result of this when the 
fourth and last movement of the dial is made the series of impulses 
coming over the vertical side of the line will actuate the vertical 
relay as usual, but its action will be to cause the rotary magnet of 
the connector to attract its armature a corresponding number of 
times, thus moving the wipers around into engagement with the 
contacts of the line of the subscriber desired. 

Assuming for the present that this line is not busy, the connector 
wipers will remain on these contacts during the conversation which 
is to follow. The final impulse over the rotary side of the line fol- 
lowing the fourth movement of the dial will actuate the rotary relay, 
R 2 R, which will close the circuit of the private magnet, P 2 M, and 



AUTOMATIC SWITCH-BOARD SYSTEMS. 719 

release the side switch to its third position. The connection is now 
completed, as this last movement of the side switch brought the 
levers, i and 2, of this switch into engagement with the right-hand 
side-switch contact, thus continuing the vertical and rotary sides of 
the line from the connector trunks, V 2 and R 2 , through the con- 
densers to the vertical rotary wipers, which already rested on the 
bank contacts connected with the vertical and rotary normals, V N 
and R N, of the desired line. 

The complete connection and all parts involved at the central 
office in making it is shown in Fig. 518. It will be seen at the 
right of this figure that the first selector of the called subscriber's 
line is involved in the connection, since the bank contacts of the 
connector employed in making the combination of circuits are con- 
nected by the vertical and rotary normals, V N and R N, to the side- 
switch of the first selector, the levers of which are in their normal 
position. It is now that the function of the bridge cut-off relay, 
C 0, may be understood. It is seen that this relay of the first selec- 
tor of the called line will, under the conditions just brought about, 
be energized, the circuit being traced from the live side of the bat- 
tery through the coil of this relay to the switch-lever 3 of the side 
switch of this first selector, thence over the private normal to the 
private wiper of the connector in question and thence to the lever 3 
of the side switch of this connector to ground. The bridge cut-off 
relay is thus energized, and as a result the vertical and rotary relays 
of this first selector are cut off, and no bridge is formed by them 
across the now completed circuit. 

The condition with respect to the talking circuit is now that shown 
in the lower schematic diagram of Fig. 513, and it only remains to 
accomplish the ringing of the called subscriber's bell. To do this 
the calling subscriber presses his ringing button which, as before 
stated, grounds the vertical side of the line at his sub-station, and 
thus energizes the vertical relay, V 2 R, at the connector in the usual 
manner. When the connector side switch moved into its third 
position, the coil of the ringing relay was brought into the local 
circuit of the vertical relay of the connector. Pressing the ringing 
button at the calling sub-station at this time therefore causes the 
energization of the ringing relay. These circuits are most clearly 
shown in Fig. 517. 

The action of the ringing relay is exactly the same as that oi an 
ordinary ringing key in a manual switch-board. It cuts off the 
line behind it and establishes connection between the limbs of the 




720 



AUTOMATIC SWITCH-BOARD SYSTEMS. 721 

called subscriber's line and the calling generator, G. The bell of 
the called subscriber is thus rung, and when he responds the con- 
versation ensues over the circuit shown in the lower diagram of 

Fig. 513. 

No mention has yet been made of the back signal relay, B S R, 
and the back release relay, B R R. The action of the back release 
relay may be best understood when the question of releasing the 
line is discussed later on. It may be said, however, concerning the 
back signal relay, that its function is to enable a subscriber who 
has been called by a toll operator to signal this operator if desired 
by means of his ringing button. 

If the called line had been busy at the moment when the connector 
wipers moved upon its bank contacts in seeking to connect with it, 
then its private normal would have been grounded. A line may 
be busy from two causes : either because it may have been sought out 
and connected with by another line, or it may have sought out and 
connected w T ith another line through its own first selector. In the 
first case the private normal would have been grounded because the 
wipers of some other connector would be in contact with the mul- 
tiple bank contact of this line, and as that connector would have its 
side switch in its third position all of the private bank contacts of 
that line would be grounded through the private wiper of that 
connector. In the second case, when the line was busy on account 
of its having originated a call, the side switch of its first selector 
would be in its third position, and therefore the private normal and 
all of the corresponding bank contacts on the group connectors 
grounded. 

As a result of this engagement by the private wiper of a con- 
nector that is seeking to connect with a busy line, a circuit is estab- 
lished which places a ground on the spring, x, of the rotary relav of 
the connector, it being remembered that the side switch of the con- 
nector has not yet moved out of its second position. The rotary 
impulse which comes over the line immediately after the final set 
of vertical impulses operates the rotary relay as usual, which by 
virtue of the ground upon the spring, x, completes a circuit through 
the release magnet of the connector, which withdraws both the vor- 
tical and rotary holding pawls from the connector shaft and restores 
it to its normal position. When in this position the act of attempt- 
ing to ring by the subscriber will, by grounding the vertical side of 
the line at the ringing button, step the shaft of the connector up 
one notch, or as many notches as the number of times the ringing 

46 



722 



AMERICAN TELEPHONE PRACTICE. 



key is pressed. This closes the off-normal switch of the connector 
and establishes a circuit from the busy test apparatus to the vertical 
side of the line, thus throwing a tone upon the talking circuit. The 
circuit by which this tone is given to the calling subscriber may be 
best seen in Fig. 519. In this, B' is a test battery of low voltage 
connected in series with an interrupter wheel, /', and a lamp, this 
circuit terminating on one side in the main battery lead, L, and on 
its other side in the off-normal switch, N, of the connector. When 
this switch is closed and the side switch of the connector is in its 
normal position the test circuit is completed, as shown in Fig. 519, 
with the result that a certain amount of interrupted current passes 
to the receiver of the calling subscriber, causing the buzz which 
signifies to him that the called line is busy. It is true that this cir- 
cuit is bridged in several places by the various relay magnets, but 




[ VR 



[ V R 3 [ V 2 R ] E BRR ] 



o- v [ RR 3 IR'RJ tR 2 R] [BSR] 

L-T r rJ£ 



OFF NORMAL SWITCH 
■■5 




FIG. 519.— DETAILS OF TEST CIRCUIT. 



sufficient current is received at the subscriber's station to produce 
the required signal. 

The clearing-out operation may be best understood from this 
figure or from Fig. 517. When the calling subscriber hangs up 
his receiver at the close of conversation both sides of his line 
are connected momentarily to earth, as has already been 
described. The vertical and rotary magnets of the first selector 
of the calling line, the second selector and the connector in- 
volved will, as a result of this, receive simultaneous current 
impulses which will cause the attraction of the armature of 
each. The releasing action is the same for all of the switches, and 
will therefore be described in connection with the first selector only. 
The closure of the contacts of the rotary magnet causes operation 
of the private magnet in a manner already described, and this closes 
one pair of contacts in the circuit of the disconnect magnet. The 



AUTOMATIC SWITCH-BOARD SYSTEMS. 723 

vertical relay, which is at the same time operated, serves to close 
another pair of contacts in the circuit of the disconnect or release 
magnet, this circuit now being traced from the live side of battery, 
through the release magnet, to the springs of the private mag- 
net, thence through the springs of the vertical magnet, and to 
ground at the opposite pole of battery. As a result the disconnect 
magnet for the first time in the operation that has been described 
receives current and withdraws the holding pawls from the shaft 
so as to allow it to return to its normal position in both vertical and 
rotary movements. 

The same action takes place at each of the other switches that 
are off normal, and both calling and called lines are thus ready to 
receive or make other connections. 

Unless means were provided to prevent it, the line of a subscriber 
might be connected with by another subscriber and left "tied up" 
by the failure of the subscriber who made the call to hang up his 
receiver and thus release the line. To guard against this occur- 
rence the back release relay on the connector is provided and con- 
nected between the rotary side of the called subscriber's line and 
battery in such manner as to be under the control of the called 
subscriber's instrument. When, therefore, a subscriber who has 
been called and not released by the calling subscriber, moves his 
dial to make a call, the back signal relay will be energized and 
thus close the circuit through the disconnect magnet of the con- 
nector switch only. As a result this switch is restored to its normal 
position, leaving the called subscriber's first selector free. 

The circuits and apparatus here described are those installed by 
the Automatic Electric Company, of Chicago, in the 6000-line ex- 
change of the Citizens' Telephone Company, of Grand Rapids. 
Mich. Practically the same circuits are installed in a plant of sim- 
ilar size at Dayton, Ohio, the system of these two plants being a 
later development and a considerable improvement over the sys- 
tems installed at Fall River, Mass., New Bedford, Conn., Chicago, 
111., and other places. 

A later development has been made, by means of which all of the 
relays at the first and second selectors are cut off from the line after 
they have served to operate their switches, this severing of their 
connections being accomplished in each case by the last movement 
of the side switch. Only the relays at the connector are left across 
the circuit during a connection, and the release of the other switches 
is accomplished at the end of the conversation by means of a circuit 



724 



AMERICAN TELEPHONE PRACTICE. 



established over the private wire connecting the bank contacts of 
one set of switches with the wipers of the corresponding selector. 
By this means a much more positive action of the various switches is 
secured, as there are fewer of them to be operated in multiple, and 
besides this a better talking circuit is afforded. 

The switch room of the Citizens' Telephone Company, of Grand 
Rapids, is shown in Fig. 520, and a closer view of the various racks 
containing the switches in Fig. 521. The switches are mounted on 
iron racks, as shown, there being one hundred first selectors on each 



f 




:..-' t V ; ....:;-. 


i " 


■ ■■■ ^ 


-• ■ - -~r: '■-.*_■ ; . ^ ■.-"■ ■ * ■ - j 



FIG. 520.-SWITCH ROOM IN GRAND RAPIDS AUTOMATIC EXCHANGE. 



rack, these occupying the first four rows. Above these are mounted 
the corresponding second selectors and connectors, these occupying 
the upper two rows. All cabling is done on iron racks overhead, in 
much the same manner as is practiced in the cabling between the 
distributing frames in the manually operated plant. 

The giving of toll service in automatic exchanges presents an 
interesting problem, and it may be said that the most approved 
method up to date is that employed at Grand Rapids, the exchange 
in this city having a very large relative amount of toll work. All 



AUTOMATIC SWITCH-BOARD SYSTEMS. 



'25 



lines, after passing through the distributing frame are led to a toll 
switching section, where they terminate in cut-off jacks. From the 
inner contacts of these jacks the two sides of the line lead to the 
automatic apparatus, where they terminate in first selectors, as 
already described. Under normal conditions, therefore, the line 




FIG. 521.— SECTION OF AUTOMATIC SWITCH-BOARD. GRAND RALTDS 

ENCHANGE. 



extends directly to the automatic apparatus, but when a jack is 
plugged at the toll section the portion of the line extending to the 
automatic apparatus is cut off, and connection is made with the 
line at the toll board as in ordinary practice. 

In this exchange the numbers from o to IOOO are not used, and 



726 AMERICAN TELEPHONE PRACTICE. 

the connection through the automatic apparatus is such that when 
the subscriber desiring to call toll places his finger in the o hole 
and pulls the dial once, his line will be continued to a recording 
operator in the toll room, and a signal displayed before that opera- 
tor indicating that her attention is required on the corresponding 
line terminating before her in a jack. In order not to confuse the 
subscribers, an extra hole is placed in the dials of their instruments, 
this being just above the o hole. This is marked "Long Distance" 
and is used instead of the o hole in calling for toll, although the 
effect produced is exactly the same at the central office as that of 
pulling the o hole. The recording operator will take the instruc- 
tions of the subscriber as to the toll connection wanted, make out 
the ticket and will tell the subscriber to hang up his receiver. After 
the required toll connection is secured by the toll operator who re- 
ceives the ticket, the connection is ordered up in the usual manner 
at the toll switching section, and connection is thus completed be- 
tween the line of the subscriber who made the call and the toll 
trunk leading to the position of the toll operator who is handling 
the connection. It will be seen that this method obviates the possi- 
bility of fraud on the part of the calling subscriber in giving a wrong 
number to which the connection is to be charged, for the connec- 
tion over w T hich he originates the call is destroyed and connection 
is afterwards made with his line through the multiple jack in the 
toll switching section. 

Incoming toll calls are handled in exactly the same manner as 
in manual practice, the local connection being ordered up through 
the switching section, thus cutting off the extension of the called 
subscriber's line to the automatic apparatus. 

With this method of handling toll service practically all the com- 
plexity of the automatic apparatus is eliminated, so far as toll work 
is concerned, and the presence of the automatic apparatus, which 
might be considered injurious to the talking efficiency in a really 
long distance connection, is thus entirely avoided. 

Some work has been done, and it seems probable that more will 
follow, in the line of applying common battery advantages to auto- 
matic systems. The works of Messrs. Malcolm C. Rorty and 
Albert M. Bullard, of the American Bell Telephone Company, of 
Boston, stand out prominently in this line. The system that they 
have perfected is at present applicable, however, only to compara- 
tively small exchanges, that is, to those exchanges small enough to 
allow each selector directly controlled by a line to make connection 



AUTOMATIC SWITCH-BOARD SYSTEMS. 727 

within itself with any other line in the exchange without the inter- 
vention of second selectors and connectors. 

Much has been done in the line of automatic telephony that might 
be described here, but attention has been given to a complete de- 
scription of the only automatic system' that has gone into wide 
commercial use rather than to several systems which, while prom- 
ising, have as yet achieved little commercial prominence. It is to 
be expected that the near future will bring forth many developments 
in this newly aroused field of telephone activity, and the advent of 
an effective party line system of working as well as of measured ser- 
vice working in its various branches, is to be confidently expected. 

It is difficult at the present time to form an accurate opinion as 
to the relative merits of the so-called automatic and manual systems. 
The writer's views on this subject, so far as formulated at the time, 
are shown in a paper entitled "The Automatic vs. The Manual 
Telephone Exchange," delivered by him before the International 
Electrical Congress at St. Louis, September, 1904. The remaining 
portion of this chapter is quoted from that paper: 

"In the manual system in its highest development, the telephone 
user has only to place his receiver to his ear and make his wants 
known, the desired connection being made at the central office by 
operators. This system may be assumed to be highly developed, 
as it has been almost universally used since the advent of telephony, 
a period of nearly thirty years. The manual system, in its present 
form, represents the consecutive work of a large number of men in 
a field of the most intense and constantly increasing activity, all 
these men striving for the best possible means of accomplishing a 
desired result. 

"In the automatic system, the central office switches are governed 
in their movements by the actions of the subscribers or users who 
desire connections and subsequent disconnections. The subscriber 
does his own work, manipulating the apparatus before him in such 
a way as to cause the switches at the central office to select, connect 
with, and afterwards disconnect from, the line of the subscriber 
desired. 

"Unlike the manual system, the automatic cannot be assumed at 
the present time to have reached a relatively high development. 
While the automatic switch-board has been in the minds of in- 
ventors since the year 1879, it is not true thai it has been put into 
considerable use until very recently. Instead, therefore, of its devel- 
opment being paramount in the minds of a large number oi prac- 



728 AMERICAN TELEPHONE PRACTICE. 

tical telephone workers, it has been fostered till lately by but few 
men, some of whom were unfamiliar broadly with the details of the 
telephone business. With a courage that must excite the admira- 
tion of all, a very few of these men have persisted, and as a result 
the telephone engineer, the operator of telephone companies, and 
last, but most important, the general public, are confronted with 
what I think is the greatest problem that has been recently before 
the telephone world: The problem of the automatic vs. the manual 
switch-board. 

"It is not the purpose of this paper to attempt to solve this prob- 
lem. The unequal degree of development of the two systems makes 
impossible a final satisfactory solution at the present time. It is 
rather to state some of its phases as they appear to me, and to make 
comment on them wherever my study of the situation has led to 
more or less positive convictions that this paper is offered. 

"A fundamental question affecting the entire problem is this: is 
it possible to make a machine serve to effect the electrical connec- 
tion of any line, in a large or small group, with any other line in 
the group, for the purpose of telephonic communication, and after- 
wards to effect a disconnection when required ? There can be, even 
at the present early stage of development, but one answer to this 
question. That it is. The automatic switch-board at Grand 
Rapids, Mich., recently selected for me ioo different lines chosen 
at random from among approximately 5000 lines centering at that 
office. Some of the subscribers called did not respond, which will 
occur in any system; and some of the lines were automatically re- 
ported busy, which is to be expected; but in no single case was the 
wrong line chosen, and in but one case was the disconnection im- 
properly effected. The verdict of a large number of subscribers 
interviewed by me in that city is practically unanimous to the effect 
that they uniformly secure their connections and disconnections 
promptly, accurately and satisfactorily. 

"I conclude, then, in view of present achievement and of that 
future progress which this must stimulate men to make, that it is 
possible for the automatic switch to perform these functions satis- 
factorily. 

"If, then, the automatic switch-board may be made to accomplish 
the commonplace connection and disconnection of lines, which form 
the great bulk of the work in a telephone exchange, is not the sys- 
tem so inflexible in its method of operation as to preclude the 
possibility of its performing the great multitude of special duties 



AUTOMATIC SWITCH-BOARD SYSTEMS. 729 

which, while not constituting the bulk of the work, are of constant 
occurrence and of hardly less importance ? I refer to such matters 
as toll connections, private branch exchange work, and to a number 
of subordinate but necessary classes of service. 

"A prominent telephone engineer has recently remarked to the 
effect that if some of the people enthusiastic on the subject of auto- 
matic switching in telephone exchanges were to visit the school for 
telephone operators maintained by the New York Telephone Com- 
pany, they would be discouraged in their efforts, as no machine 
could ever be made to perform the many and varied functions that 
it was necessary to teach these young ladies before they became pro- 
ficient telephone operators. This seems to be a statement that has 
very little to do with the real automatic problem. It should never 
be required that the machine shall do the same work that is de- 
manded of the girl, nor do it the same way. That is manifestly 
impossible, for no machine can ever be endowed with intelligence. 
(It may be that you will say that there are some telephone girls 
similarly affected.) Since the very reason for the existence of the 
automatic exchange is to do away largely with the operator, it fol- 
lows logically that whatever intelligence is to be applied to the 
making of the ordinary connection between two lines, it shall be 
that with which the subscriber desiring to make the connection is 
endowed. Here is a fundamental difference between the two sys- 
tems which must always lead to different modes of operation. 

"The real functions that the automatic switch-board should be 
required to do automatically are those relating to the ordinary 
routine work of connecting and disconnecting subscribers' lines 
under the control of the calling subscriber. When some act need- 
ing intelligence at the central office is required, then let an operator 
supplement the work of the machine. To condemn the automatic 
switch because it will not perform all of the special requirements 
without the aid of human intelligence, is just as unfair as to con- 
demn a linotype machine because it cannot digest one of Steinmetz' 
equations. My mind has gradually changed upon this point until 
the doubt now exists as to whether the automatic system, wisely 
supplemented by operators, is not even more flexible than the man- 
ual. It is the ease with which the personality of the operator may 
be introduced into the automatic system, and also the ease with 
which certain of the purely automatic functions may be varied by 
mere changes in the circuit, or in the mechanical relation of the 
parts, that makes this doubt exist. 



730 AMERICAN TELEPHONE PRACTICE. 

"Of course, there are many phases of traffic and service that are 
yet to be worked out for the automatic system, but apparently the 
longer one studies the problem the more nearly he becomes con- 
vinced that the automatic system is sufficiently flexible, with the 
interjection of human intelligence when necessary, to make possible 
the solution of practically all of the problems of service. 

"So far as I am aware, selective-signal party line working has 
never been accomplished commercially with automatic systems, I 
believe that the reason for this is solely the fact that automatic tele- 
phony is yet new. I have recently seen a plan whereby any ordi- 
nary number of stations can be selectively operated on a party line 
with practically no other added complication either at the central 
office or at the subscribers' stations than that which is added to the 
apparatus of an individual line manual system to adapt it to the 
same class of party line work. While the automatic party line is 
not yet developed to the extent of actual commercial use, it is 
entirely feasible, and will not be one of the controlling factors in the 
solution of the problem: automatic vs. manual. 

"I have looked into the subject enough to believe that what 
is true of the party line problem is true of the common battery 
problem, and also of the measured service problem, whether the 
measuring of the service is accomplished by collecting coins or 
tokens at the subscribers' stations or by operating counting devices 
either at the sub-stations or at the central office. There is undoubt- 
edly a vast amount of work yet necessary before these features are 
commercially incorporated in working apparatus in an entirely sat- 
isfactory manner. I merely say that my study has shown me that 
no insurmountable obstacles exist that would prevent the successful 
establishment of party line, common battery and measured service 
working. 

"These statements do not greatly help the man who is to-day 
casting about in making a choice between the automatic or the 
manual system for present use. It is not, however, with the present 
alone that we are concerned. We must plan and build for the 
future; and the remarks just made are given merely as bits of con- 
tributory evidence as to what developments may be expected. 

"Having seen that the thing is possible, that it seems from a tech- 
nical standpoint to be able to do what is wanted, another question is: 
do the subscribers like it? 

"The evidence all seems to point in one direction. They do. 
At Grand Rapids, Mich., ninety-five per cent, of a large number of 



AUTOMATIC SWITCH-BOARD SYSTEMS. 731 

subscribers interviewed by me liked it better than common battery 
manual service; four per cent, did not care much one way or the 
other, and one per cent, liked the manual system better. At Fall 
River, Mass., where the system has been in use for a much longer 
period, the verdict was quite the same in effect. Evidence from 
other cities where automatic service is being tried seems to agree. 
It must be said in fairness, however, that at Grand Rapids the 
mass of subscribers is leavened by the presence of a large number 
of stockholders in the local company. Again, there is in that city 
much civic pride in the system, as telephone people come from all 
parts of the country to inspect the plant. Still again, the delight 
of the subscribers may be similar to that of a child with a new toy, 
but this can hardly be true, because of the fact that the exchange 
at Grand Rapids has been in service for a period of nearly nine 
months and is carrying a very large business load, so that if the 
people were not actually getting satisfaction they would probably 
know it. The new toy idea is also apparently disproven by the 
condition at Fall River and New Bedford, where the service has 
been maintained for several years and seems to be much liked. 

"The question also naturally arises : is not the automatic switch- 
board and necessary subscribers' mechanism too complex to be 
maintained in proper working order without undue cost? It is 
perhaps too early to decide this question. There is not enough 
evidence one way or the other. Judging from the past, however, 
the tendency of industrial achievement seems to be toward auto- 
matic methods. As examples, take the arts of printing, of weaving 
and the use of machine tools. 

"Summing up, therefore, the statements already made, the auto- 
matic system is not only a possibility, but is actually here. With 
the interjection of human intelligence to supplement it in perform- 
ing certain functions, it seems to be as flexible as the manual. Party 
line, common battery and measured service working, while not yet 
achieved commercially, so far as I am aware, seem to be well within 
the grasp of those who are doing the development work. The 
public seems to like it, and we do not know whether it is too complex 
or not. 

"It will be noted from the foregoing that the idea of having the 
central office apparatus perform all the phases of telephone service 
is apparently not tenable. Many of those who have advocated it 
in the past have abandoned it and are introducing human aid in the 
performance of some of the functions. This being true, a certain 



732 AMERICAN TELEPHONE PRACTICE. 

number of operators are and will be needed in automatic exchanges. 
This tends to destroy in some degree the primary object of the 
automatic system — the doing away with operators. We have seen 
many papers bearing on each side of this question, to the effect 
that the salaries of the operators were or were not to be eliminated; 
that retiring rooms, matrons, operators' luncheons, etc., were or 
were not to be done away with. These items of expense will 
probably exist to some degree in all large automatic exchanges. 
That they will be greatly reduced is without question, but whether 
or not they are reduced to such an extent as to offset other sources 
of expense introduced by the employment of automatic apparatus, 
is a problem yet to be solved. 

"What are some of these sources of expense that tend to offset 
the reduction in operators' salaries and expenses coincident there- 
with ? Taking the automatic system as a whole, we find that it is 
considerably higher in first cost than the manual system, and as- 
suming that interest and depreciation are at the same rate in each 
case, this shows to considerable disadvantage for the automatic 
system in the annual charges due to these items alone. 

"For an exchange of 5000 lines served by one office, the cost of 
automatic equipment, including telephones, may be taken at $35 
for each individual line. In manually operated exchanges the cor- 
responding cost is not far from $25 per line. The difference be- 
comes greater, that is, more in favor of manual, for smaller offices, 
and smaller or less favorable to the manual in larger offices. 

"Whether or not the depreciation on automatic apparatus should 
be taken at a higher rate than that on the manual, is a question 
that we have not at present sufficient data or information to deter- 
mine. It is true' that in the automatic switch-board the flexible 
cord nuisance found in all present forms of manual switch-board 
apparatus is largely eliminated. It is also true that the automatic 
apparatus is more complicated and requires greater care in its main- 
tenance; but whether, if both systems are maintained with reason- 
able care, the automatic will show a greater rate of depreciation than 
the manual, I am not at all certain. Much of the depreciation in 
manual telephone apparatus is due, not to the fact that the apparatus 
wears out, but rather to the fact that it is rendered obsolete by new 
inventions. That the same will be true in the case of automatic 
apparatus cannot be doubted, but it is a good point to bear in mind 
that if telephonic development should point toward automatic 
apparatus to the exclusion of manual, and should prove the supe- 



AUTOMATIC SWITCH-BOARD SYSTEMS. 733 

riority of automatic, then the highest developed and newest manual 
apparatus will depreciate greatly in value by that fact alone. It 
does not seem unreasonable, therefore, to place the rate of deprecia- 
tion on both manual and automatic apparatus at about the same 
figure. 

"In point of maintenance the advantage must be conceded to the 
manual. This is certainly true at present with regard to both the 
central office and the subscriber's station apparatus. No good rea- 
son is apparent why it should not always be true. Automatic 
apparatus is especially at a disadvantage at the subscriber's stations, 
and it is really at this point that the automatic system seems to in- 
volve a poor engineering feature. The tendency of telephone de- 
velopment in regard to sub-station apparatus has been until lately 
along what seemed to be unquestionably good engineering lines. 
The sub-station equipment has been gradually simplified, the bat- 
tery has been removed, as has also the magneto generator, and the 
instrument has been reduced to the simplest fundamental parts. 

"Automatic telephony as at present developed for large work 
takes a step backward by reintroducing the local battery. That this 
is disadvantageous no one can deny, but on the other hand, it must 
be pointed out that the disadvantage is by no means as great as it 
would have been several years ago because of the fact that dry 
batteries have recently come into almost universal use for this kind 
of work, and are far superior, all things considered, to anything 
heretofore available. 

"The disadvantage of local batteries, while mitigated, is still pres- 
ent, and is real ; but, taking the automatic system as we have reason 
to believe it will exist in the future with no local batteries, it will 
still possess, as far as we are able to see, a more or less complicated 
impulse transmitting device, by means of which the subscriber will 
be able to direct the movements of the switches at the central office. 
Complexity, not only of mechanism, but of function, is thus intro- 
duced at the subscriber's instrument, and this seems to be an in- 
herent disadvantage to all present schemes of automatic exchange 
working. This, of course, is another factor that must be weighed 
in considering the relative economies of the two proposed methods. 

"There is a point that I have not yet seen mentioned in print, 
which, under certain cases, seems to be of great importance. This is 
the matter of trunking between two or more automatic offices in such 
cities or communities as naturally demand, by the distribution of 
their subscribers, more than one office. Tt is true that the present 



734 AMERICAN TELEPHONE PRACTICE. 

automatic switch-board seems to be capable of properly handling 
this condition if the requisite number of trunk lines between the 
offices are provided. At first thought it seems that the number of 
trunks required between offices for a given amount of traffic might 
be somewhat less in the case of the automatic than in the case of 
the manual system, on account of the immediate disconnection and 
release of the trunks; in the automatic, upon the hanging up of the 
receiver of the calling subscriber. Further consideration, however, 
will show that there is very little difference in the time the trunk is 
held busy in the two systems, the length of actual conversation 
being assumed to be the same in each case. The reason for this is 
that, while the automatic gains in this respect in the release, it loses 
something in the making of the connection, because in the case of 
the automatic, the trunk is selected with the first movement of the 
dial by the subscriber, and the length of time that the trunk is held 
busy, therefore, must include the time during which the subscriber 
is setting up his own connection ; whereas, in manual boards a trunk 
line begins to be busy at the time when the B operator picks up the 
incoming trunk plug and designates its number to the A operator. 

"So far there seems to be little difference between the systems in 
this respect. 

"The bearing on the trunking problems of the relative efficiencies 
of different sized groups of trunks between offices does not, how- 
ever, seem to have been weighed by many in considering the ques- 
tion of automatic vs. manual exchanges. When sufficient trunks 
are provided between offices to handle business on the so-called 
'no delay' basis, it is known that a large group of trunks will handle 
very much more business per trunk line than a small group. For 
instance, when there are only ten trunks in a group between offices, 
it is a well-established fact that slightly less than eighty calls per 
trunk per day may be handled. If, however, the group be increased 
to ioo trunks, as many as 145 calls per trunk per day may be 
handled. This is an increase of considerably over eighty per cent, 
in actual trunk efficiency. In the present automatic system, group 
the trunks as you may, it is inherently true that the efficiency of 
the trunks is reduced to that of a group of ten. I do not mean by 
this that it is not possible to place as many trunks as desired be- 
tween any two offices, but that any subscriber has access to ten 
trunks only in order to secure a connection to any other office. It 
is true that some other subscriber may have access to another ten, 
or to the same ten, but no one subscriber can reach more than ten. 



AUTOMATIC SWITCH-BOARD SYSTEMS. 735 

This seems to be a grave objection to the use of automatic systems 
as at present developed, in those communities where several offices 
must be employed and where traffic is such as to demand a large 
number of trunks between offices. The remedy to this is obviously 
that of giving the subscriber the chance to select his trunks from 
larger groups. This, I take it, is one of the problems that need 
serious consideration in adapting the automatic system to very large 
communities, It does not enter seriously in single office work. 

"In all that I have said I have attempted to take the very practi- 
cal view of the engineer, and fundamentally that view must always 
compare systems with the intent of selecting a means of doing w T hat 
is required well enough for the smallest price. From the strictly 
engineering view one does not take into account relative populari- 
ties of mere ways of accomplishing results. But this is necessary in 
such a case, for there are features of the automatic system which may 
make it so popular as to force upon the owners or prospective own- 
ers of telephone industries a serious consideration of the doctrine 
of expediency. This is' by no means the least of the important 
things to consider. 

"I expect to be criticised because I have not solved the problem. 
It cannot now be solved any more than the question of alternating 
versus direct-current transmission could be decided when we first 
were brought to realize that there was an alternating versus direct- 
current transmission problem. My object has been to state the 
problem as I see it, and I hope »that in doing this something may 
have been accomplished toward clarifying it." 



CHAPTER XXXVI. 
INTERCOMMUNICATING SYSTEMS. 

Two general plans of installing interior telephone systems for giv- 
ing service between the various departments of a business establish- 
ment may be followed : One of these is to install a switch-board at 
some central point to which all the lines radiate, and at which they are 
connected as desired by an operator. In following this plan the switch- 
boards and instruments used may be of any of the types already out- 
lined for use in small exchanges. The second plan involves the use of 
what is called an intercommunicating or house system, in which the 
instrument at each station is placed on a separate line, the line belong- 
ing to each station passing through all of the other stations. By 
means of a simple switching device arranged in connection with each 
telephone, the party at any station may at will connect his telephone 
with the line belonging to any other station and call up the party at 
that station without the intervention of an operator. This involves 
the necessity of running at least one more than as many wires as there 
are instruments in the exchange through each one of the stations; 
and the simplest way to do this is to run a cable having the requisite 
number of conduits through each of the stations, all of the conductors 
in the cable being tapped off to the switch-contact points on each tele- 
phone. The connections for a system having four stations is shown 
in Fig. 522. Each of the telephone sets embraces the ordinary talk- 
ing and calling apparatus switched alternately into circuit by the 
ordinary form of hook-switch. These instruments differ in no re- 
spect from the ordinary magneto exchange telephone. 

Connected with one of the binding posts, b, of each instrument is 
the pivot of the lever, L, which lever is adapted to slide over the 
buttons, 1, 2, 3, and 4, arranged in the arc of a circle beneath. Each 
button on each telephone is connected with a line wire, 1, 2, 3, or 4, 
bearing the same number as the button. The binding post, b', on 
each instrument is connected with the common-return wire which 
runs through the same cable as the line wires. During the idle 
periods of each instrument the lever is kept on the button bearing 
the same number at that station. This button is usually called the 
home button, and is for convenience placed at the extreme left of 

736 



INTERCOMMUNICATING SYSTEMS. 



737 



the row of buttons on each instrument. The apparatus as shown 
represents the condition when station I. is about to call station IV. 
For this purpose the party at station I. has moved the lever, L, from 
its home button to button No. 4, thus connecting the instrument at 
station I. with the line belonging to station IV. When the genera- 
tor at station IV. is operated, the current flows from binding post, b, 
to the common-return wire to the binding post, b\ at station IV., 
thence through the generator and call-bell at that station to binding 
post, b, and to lever, L, whence the return is made by line wire, 4, 
to the lever, L, and the binding post, b, at station I. When the re- 
ceivers at both stations are raised the talking apparatus is thrown 




FIG. 522.-CIRCUITS OF ORDINARY HOUSE SYSTEM. 



into the circuit over which the calling current was just sent, and the 
parties converse over the common-return wire and line wire No. 4. 
Had station IV. called station I., then the talking and ringing 
would have been done over the common-return wire and line No. 1. 
The great drawback to the system of wiring shown is, however. 
that the lever at the calling station must always be moved back 
to the home button when a conversation is finished. If this is not 
done the instrument at that station will be left switched upon the 
wrong line, and will not respond to a call sent over its own line from 
another party. Moreover, when anyone calls a party on the line to 
which these two stations are left connected, both bells will ring, thus 
producing much confusion. To illustrate this : if after station 1. had 
47 



738 AMERICAN TELEPHONE PRACTICE. 

called station IV. he had left his switch lever, L, in the position 
shown, station II. could not call station I. because the instrument at 
station I. would no longer be connected with line No. I. Should 
station II. attempt to call station IV., the bells at both stations I. and 
IV. would ring because both of those instruments are connected with 
line No. 4. 

Frequently, instead of using a rotary switch, an ordinary plug and 
cord are used in place of the switch lever, while the buttons are 
replaced by simple spring-jacks into which the plug may be inserted. 
In Fig. 523 is shown STich a system, where plug, P, in each case 
takes the place of the lever, L, in Fig. 522. Ten line wires are 
shown in this figure, each connected with ten spring-jacks on each 
of the telephone instruments ; the wiring of but five instruments is 
shown, this being a sufficient number, inasmuch as all are connected 
to the circuits in the same manner. This system is for common 
battery work, a single battery located at any convenient point being 
used for supplying both talking and calling current to all of the 
stations. This battery is connected across the common-return and 
battery wires, which are common to all of the stations, and which 
are placed in the same cable as the line wires. Connected between 
the common return wire and the line wire bearing the same number 
as its station is an ordinary vibrating bell, the circuit through which 
is broken when the receiver is removed from its hook. By pressure 
upon the key, k, at any station, circuit may be completed from the 
common-return wire through the battery to plug, P, of that station, 
and therefore if this plug is inserted into the jack belonging to any 
other station the pressure upon this key will cause the bell to sound 
at that station. In this way a call may be received or sent. When 
the hook-switch is raised the transmitter of a station is connected 
between the battery wire and common-return wire, so that all of the 
transmitters at the stations in use take current from the same battery 
in multiple. 

In order to reduce cross-talk between two or more pairs of sta- 
tions which happen to be communicating at the same time, the small 
impedance coils, c c, are placed in each side of the transmitter cir- 
cuit at each station, and between their terminals, as shown, is 
bridged at each station a condenser, C, so as to afford a local circuit 
for the vibratory currents set up by the transmitter. The coils, c, 
tend to prevent the fluctuations in current produced in any trans- 
mitter from backing up through the battery wire and common- 
return wire into the local circuits of the other transmitters. Flue- 



rlMl 



T^TUifefc? 








10 



a 



2 
10 



-^jasaci 



S 



^ _^^ _ 




739 



■40 



AMERICAN TELEPHONE PRACTICE. 



tuations produced in the local circuit of any transmitter act in- 
ductively through the induction coil, /, upon the talking circuit 
containing the receiver, the circuit being completed between two 
stations by the common-return wire and the wire of the station t..at 
has been called. This arrangement necessitates the removal of the 
plug when through talking, as otherwise both of the stations con- 
nected would be rung up when either of the stations was called. 

The Holtzer-Cabot Electric Company has overcome the difficulty 
due to the subscriber calling leaving his switch lever in the wrong 
position, by the apparatus shown in Fig. 524, this device being the 




FIG. 524.— NESS AUTOMATIC SWITCH FOR HOUSE SYSTEMS. 



invention of Mr. T. W. Ness. The arrangement is such that when 
the subscriber hangs up his receiver the switch arm, which is under 
the influence of a spring, will be automatically released and will fly 
back to the home position without his volition. In the figure the 
switch-restoring mechanism is mounted on the inside of the cover 
of the box, the switch lever itself being mounted on the opposite 
side. The lever, L, at each station, shown in diagram in Fig, 525, 
is adapted to slide over the buttons, 1, 2, 3, and 4, as in the systems 
already described. The curved contact-piece, D, is so arranged that 
the lever will not normally engage it, but by pressure upon the 



INTERCOMMUNICATING SYSTEMS. 



741 



handle of the lever it may be brought into engagement with the con- 
tact. 

Referring again to Fig. 524, H is the hook-switch adapted to per- 
form the ordinary functions of connecting the calling and talk- 
ing apparatus alternately in the line circuit. The switch lever is 
mounted upon the shaft, A, which may be seen passing through the 
front board of the box and which carries a ratchet-wheel, E, of 
hardened steel. A coiled spring around the shaft tends to rotate it 
so as to bring the lever always to the home position. F is a sliding 
pawl normally held in its lower position by a coiled spring sur- 
rounding it. This sliding pawl serves to hold the lever, L, in any 




FIG. 525.— CIRCUITS OF HOLTZER-CABOT SYSTEM. 



position to which it has been rotated, by the engagement with the 
teeth of the ratchet-wheel, E. Upon the short arm of the hook- 
switch is pivoted a dog, G, adapted, when the receiver is placed upon 
the hook to engage a notch in the pawl, E, and lift it out of engage- 
ment with the ratchet-wheel. This allows the spiral spring to re- 
turn the switch lever to its right-hand position in contact with the 
home button. After raising the pawl out of the notch on the 
ratchet-wheel the dog slips out of the notch on the pawl, thus allow- 
ing the latter to return into contact with the ratchet-wheel, in order 
to be ready for the next use of the telephone. In order, however, 
that the pawl may not engage the ratchet before the lever. A. has 



42 



AMERICAN TELEPHONE PRACTICE. 



fully returned to its normal position, a second dog, /, is provided, 
which is pressed by a spring so as to occupy a position under the 
pin, p, carried on the pawl, thus holding it out of engagement with 
the ratchet-wheel until the rotation of the lever is nearly completed. 
At this point' a cam on the under side of the ratchet-wheel pushes 
the dog, /, out of engagement with the pin, p, and thus allow T s the 
pawl to drop into position against the ratchet-wheel. It will be 
seen that this device accomplishes with certainty what the memory 
of the telephone user could not be relied upon to do. This entire 




FIG. 526.— HOLTZER-CABOT DESK SET. 



mechanism is well constructed, all of the parts subject to wear being 
of hardened steel. 

The diagram of circuits given in Fig. 525 shows a system of wiring 
for four stations equipped with this automatic return switch, this 
system being operated with the common calling battery and with 
local batteries at each station for talking purposes. 

Fig. 526 shows such a device applied to a desk telephone and also 
shows a method of wiring adopted in such systems. A cable hav- 
ing a sufficient number of conductors is run through each of the 



INTERCOMMUNICATING SYSTEMS. 



743 



stations to be served, and each conductor is brought out to a con- 
nector in a terminal box, as shown. To these terminals are also 
secured the conductors of the cable leading to the telephone instru- 
ment and switch. 

There is another way than by the use of the spring-actuated re- 
turn switch, of obviating the difficulty mentioned in regard to sub- 
scribers forgetting to switch their instruments in connection with 
the home line after use. This involves no mechanical complications 
whatever, the circuits being so arranged that it makes no difference 
whether the subscriber leaves his switch lever or plug in connection 
with some line other than his own or not. This is accomplished bv 



STATION 11*1 




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STATION M«4 


STATION N«5 


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FIG. 527.— MAGNETO INTERCOMMUNICATING SYSTEM, 

CIRCUITS. 



METALLIC 



bridging the call-receiving bell, which in this case should be high 
wound directly across the line circuit belonging to the station at 
which the bell is placed. This bell remains permanently so con- 
nected, the subscriber's switch arm or plug serving only to connect 
the talking and call-sending apparatus with the line of the party 
with whom it is desired to talk. An arrangement of this kind using 
individual metallic circuits for each line is shown in Fig. 527. In 
this there are five lines running through five separate stations and 
the call-receiving bell, B, of each line is permanently bridged across 
the line at that station bearing the same number as the line. Two- 
point spring jacks are provided at each station for each line, and the 
subscriber's telephone set and generator may be switched into the 



744 AMERICAN TELEPHONE PRACTICE. 

circuit of any line by means of the plug in which the set terminates. 
Thus, if a party at station No. i desired to call station No. 5 the 
plug at station No. 1 would be inserted in jack No. 5 and the gen- 
erator operated. This would ring the bell at station No. 5, and the 
subscriber at that station would respond by inserting the plug in 
his own home jack. When through talking if the subscriber at 
station No. 1 left his plug in connection with line No. 5 no harm 
would be done, as other parties could operate the call bells of either 
line No. 1 or No. 5 just as well with the plug inserted as if it were 
withdrawn. 

The system of Fig. 527 shows individual metallic circuit lines 
for each station, and this practice is one that is well to follow in the 
installation of house systems, especially if a considerable length of 
cable is involved. By this means, and this means only, complete 
freedom from cross-talk may be obtained. The expense of run- 
ning two wires instead of one to each station is not as serious in 
point of cost as might be supposed. For interior work it is com- 
mon to use ordinary switch-board cable for the wiring of such sys- 
tems, the color code in the cabling being of great convenience in 
connecting up the station when originally installed. 

As a rule, about twenty stations have been considered the greatest 
number that may be satisfactorily served by an intercommunicating 
system, and when a greater number of stations is to be installed it 
has been thought better to use a central office provided with a 
switch-board with an operator in attendance. There is no inherent 
reason, however, why the number of stations should be limited to 
twenty, and where stations are close enough together so that the 
cost of wiring is not prohibitive, a much greater number than this 
may be used. 



CHAPTER XXXVII. 

THE TELEPHONE RELAY OR REPEATER. 

One of the most attractive fields of research and invention in 
telephony has been that of the telephone relay or repeater. It has 
been very natural to suppose that the principle of repeating now- 
used so successfully and extensively on long telegraph lines could 
be used with equal advantage on long telephone lines. The idea 
is very simple, and involves merely the placing of a microphone 
contact in operative relation with the diaphragm of a receiver con- 
nected in the first line circuit, and causing the changes produced 
in the resistance of this contact, when acted upon by the receiver 
diaphragm, to vary the strength of a current in a local circuit, which 
circuit would in turn act inductively on the second line wire with 
reinforced energy. 

This method is outlined in Fig. 529, where A is the transmitting 
station, being provided with a transmitter, T, battery, B, and in- 





FIG. 529.— SIMPLE RELAY CIRCUIT. 



duction coil, /. L is the transmitting line, having connected in 
its circuit the coil of a receiver, M. D is the vibrating diaphragm 
of this receiver against the center of which rests a pair of micro- 
phone contacts, which may be the same as those in the Blake trans- 
mitter, or of any other type. This microphone contact must be 
so arranged with respect to the receiver diaphragm that any vibra- 
tions of the latter will be imparted to the former, thus causing them 
to vary their resistance in exactly the same manner as if acted upon 
directly by sound waves. The microphone contact, C. servos to 
vary the resistance of a local circuit containing a battery, B'. and 
the primary of an induction coil, /', at the relay station, and the 
receiver, R, at the receiving station, A'. Any changes in current 

745 



746 AMERICAN TELEPHONE PRACTICE. 

in the local circuit at the station, A, produced by the operator at the 
transmitter, T, will induce alternating currents in the line, L, in the 
ordinary manner, which will cause the diaphragm, D, to vibrate as 
in everyday practice. The vibrations of D will be imparted to 
the microphone contact, C, which will produce changes in the 
current flowing in the local circuit at the relay station correspond- 
ing to those taking place in the local circuit at station, A. These 
changes will act inductively on the line circuit, L', in the ordinary 
manner, the receiver, R, finally reproducing the sound. 

Such an arrangement as this will do its work well, but it is 
quite evident that the transmission may be affected only in one 
direction. When it is desired to transmit from station A' to A, 
a separate circuit would ordinarily have to be used. Much diffi- 
culty was experienced in making a two-way repeater, for no auto- 
matic switch could be arranged which would bring about the 
changes of circuit required when the transmitting station desired 
to become the receiving. Many attempts were made to associate 
two relays with the line circuits in such manner that no interfer- 
ence would occur. The difficulties involved in this were, however, 
great ; and chief among them was the fact that two relays when 
associated with the same pair of lines would almost invariably 
set up a singing sound, due to the mutual action between the two; 
for instance, a slight vibration of the diaphragm of one relay 
would produce changes in current in the local circuit, which would 
act upon the diaphragm of the other relay, producing another 
change of current, which would in turn react upon the first relay. 
This action is somewhat analogous to that produced by holding the 
earpiece of a telephone receiver directly in front of the mouthpiece 
of a good granular-carbon transmitter ; the singing or shrieking 
noise set up when a proper adjustment is obtained in this case 
being due to. the fact that the sound waves set up by the receiver 
diaphragm act upon the transmitter diaphragm, which in turn 
causes currents to flow through the receiver coil, causing its dia- 
phragm to vibrate still more strongly. This defect, however, was 
finally overcome, several inventors having produced two-way re- 
lays which were successful in so far as they would operate in either 
direction with equal facility, and with a fair degree of clearness. 

One of these systems, devised by Edison, is shown in Fig. 530, 
in which A and A' are the telephone stations, each arranged in the 
ordinary manner. M is the magnet of the relay receiver, the coil 
of which is included in a local circuit containing the secondary, 3, 



TELEPHONE RELAY OR REPEATER. 



747 



of an induction coil. The primary winding of this coil is divided 
into two parts, I and 2, these parts being connected together in 
one side of the combined circuit of the two lines, L and L' . Be- 
tween the juncture of these two primary coils and the opposite side 
of the line is connected the secondary coil, 4, of an ordinary induc- 
tion coil. The primary coil, 5, of this latter induction coil is con- 
nected in a local circuit containing the relay microphone contact, 
C, and the local battery, B" . Assuming that station, A, is for 
the time being the transmitting station, currents set up in the line 
circuit, L, will divide at the relay station, part passing through the 
coils, 1 and 4, and back to the transmitting station, and the other 
part passing through the primary coils, 1 and 2, in series and to 
the receiving station direct. The current passing through the coil, 
4, will, however, under ordinary circumstances, be by far the greater 
on account of the high resistance of the long line, L '. The cur- 
rent passing through the coils, 1 and 2, however, will act induct- 
ively upon the coil, 3, thus causing currents to flow through the 




FIG. 530.— TWO-WAY RELAY CIRCUIT. 



coil on magnet, M, and produce changes in the contact resistance 
of the microphone. These changes will cause fluctuations in the 
current in the local circuit, which fluctuations will act through 
the primary coil, 5, upon the secondary coil, 4, and cause 
currents of considerable comparative strength to flow in the line 
circuit, L', to the receiving station, A'. It is obvious that as 
the various circuits at the relay station are symmetrically connected 
with respect to the two lines, L and L', the station. A, may in turn 
serve as the transmitting station. No reactive effect between the 
relay transmitter and receiver will in this ease be produced, and 
the means for preventing this forms the most interesting portion 
of this invention. Whatever currents are set up in the coil. 4. by 
the action of the microphone contact, C, will divide equally bet ween 
the primary coils, 1 and 2, passing through them in opposite di- 
rections. These coils will therefore act differentially u\\m the coil. 



748 



AMERICAN TELEPHONE PRACTICE 



3, and their effects will be neutralized. No current will be caused 
to flow in the circuit containing coil, 3, and the relay magnet, M, 
and therefore no reactive effect will be produced upon the trans- 
mitter. In other words, any current flowing in either line circuit 
will induce currents in the local circuit containing the magnet, M, 
while currents set up in the coil, 4, by virtue of currents flowing 
through the magnet, M, will produce no effect in turn upon the 
coil, 3. 

A great many improvements have been made in the mechani- 
cal construction of the telephone relay, but with few exceptions 
they have embodied only the idea of combining an ordinary trans- 
mitter with an ordinary receiver. In 1897, however, a relay was 



c:: ^ 




FIG. 531.-ERDMAN REPEATER. 



devised by Mr. A. W. Erdman, and is shown in Fig. 531, and 
embodies probably the most radical departure in the structure 
of telephone repeaters of all since the first was produced. In this 
figure L is the transmitting line, and L 2 the receiving line. H is 
the diaphragm of the receiving instrument and is used to operate 
the balanced valve, V 2 , which by its motion to and fro varies the 
flow of an otherwise constant stream of air flowing through the 
chamber, C. This chamber is covered by a flexible diaphragm, D, 
which is caused to vibrate by the changes in pressure within the 
chamber produced by the motion of the valve, V 2 . The diaphragm, 
D, serves to operate a microphone, T, which in this case consists 
of the variable resistance button of the solid-back transmitter. R 
is a reservoir containing compressed air, and V a reducing valve 



TELEPHONE RELAY OR REPEATER. 



749 



by which the amount of air escaping through the chamber may be 
regulated. In the balanced valve, V 2 , £ is a flexible diaphragm 
and A a movable portion which controls the outlet. The centers 
of the diaphragm, E, and of the valve plate, A, are connected by 
the rod, F, to the center of the receiving diaphragm, H. The bal- 
ancing of the valve, F 2 , renders it extremely sensitive, so that it may 
be set in motion by the delicate movements of the diaphragm, H. In 
operation, the vibrations of the diaphragm, H, caused by currents in 
the transmitting line, L, cause the balanced valve, V 2 , to vary the 
opening of the air outlet. This produces changes in pressure with- 
in the air chamber under the diaphragm, D, which cause that dia- 
phragm to vibrate and thus actuate the microphone in the usual 




FIG. 532.— STONE REPEATER. 



way, thus causing currents to flow in the receiving line, L 2 , in the 
usual way. No reports have been made public concerning the 
results obtained in actual practice with this repeater, but it seems 
that it may be a step toward the solution of this difficult problem. 
Instead of employing the mechanical connection commonly used 
between the diaphragms of the transmitting and receiving mech- 
anisms, Mr. Erdman has, in his current of air or gas, chosen one 
of the most delicately subtile mediums known. 

Another relay, devised bv Mr. John S. Stone of the American 
Rell Telephone Co., is shown in Fig. 532. This relay differs in 
the essentials of its construction from those of the older type only 
in that its entire working parts are inclosed in a vacuum chain- 



750 



AMERICAN TELEPHONE PRACTICE. 



ber. The repeater, together with the circuits of the two connected 
lines, is shown in this figure, in which T is the transmitter of the 
sending station and t the receiver of the receiving station. These 
are connected with the repeater by lines, L 4 and L, respectively, 
the line circuits being associated with the repeater circuits by in- 
duction coils, I and I 2 , in the usual manner. B is a polarized 
electromagnet whose poles are in proximity to the diaphragm, D. 
C is the variable resistance button of a solid-back transmitter, the 
front electrode of which is rigidly secured to the center of the 
diaphragm, D, while the back electrode is rigidly secured by means 
of a cross-piece to the frame, A, which also supports the diaphragm, 
D, and the electromagnet, B. £ is a bell jar closely fitted to the 
base, b, by an air-tight joint. The air from within the chamber 
may be withdrawn by the pipe, P, attached to an air pump. It is 




FIG. 533.— COOPER-HEWITT REPEATER. 



said that the removal of the air from within the chamber brings 
about a decided improvement in the operation of the repeater. 
Concerning the results obtained, Mr. Stone says, "The messages 
automatically transferred by it from one circuit to another are 
reproduced in the receiving telephone of the second circuit with 
a well-defined gain in volume or loudness, and without any sub- 
stantial distortion or offsetting loss in clearness of articulation." 
If this claim is borne out in practice, the production of this relay 
should prove a step of some importance in the matter of long- 
distance telephony. 

It has seemed plausible that very feeble currents received at the 
relay station would, by virtue of the delicate action of the micro- 
phone, be able to produce comparatively large changes in resistance 
of the local relay circuit associated therewith, and that these changes 
in resistance would produce correspondingly great changes in 



TELEPHONE RELAY OR REPEATER. 751 

the current of the local battery at that station, which changes would 
act inductively on the second line wire with perhaps as much 
energy as that imparted to the original circuit. As a matter of fact, 
however, no gain in the volume of transmission has ever been com- 
mercially effected by this method. The telephone repeater may be 
made to work perfectly on ordinary lines, but it has not shown its 
ability to transmit speech between two distant points any better 
than or quite as well as could be done by direct transmission with- 
out the use of the relay at all. The amount of energy received by 
the electromagnet of the relay is so exceedingly small that it does 
not seem capable of producing the desired mechanical effect upon 
the microphone contact. Something more subtile in its action than 
the microphone contact, a medium devoid of all mechanical inertia, 
is evidently needed. 

A step that is apparently in the right direction has recently been 
made by Mr. Peter Cooper Hewitt, of mercury lamp fame. He has 
found that the resistance of the mercury vapor column is extremely 
sensitive to changes in the magnetic flux within which it lies. He 
therefore brings the mercury vapor tube, A, Fig. 533, within the 
field of an electromagnet, B, so that changes in the field set up by 
the latter will produce changes in the resistance of the mercury tube. 
The magnet, therefore, forms the receiving device and the tube the 
resistance-varying device, when connectd as a telephone repeater. 
This is certainly of great scientific interest. 



CHAPTER XXXVIII. 
WIRE FOR TELEPHONE USE. 

The wires in use in telephone work are, at present, of copper 
and iron exclusively. Aluminum will probably, as the price of its 
manufacture is cheapened, come into extensive use, and it will not 
be surprising if it eventually supersedes both copper and iron for 
all except very long distance service. Iron possesses a slight ad- 
vantage over copper on account of its tensile strength, and a very 
decided advantage in point of first cost, but in all other respects 
copper is vastly superior. 

The tensile strength of a wire is its ability to resist a pulling stress 
and the amount of tensile strength is usually expressed in the num- 
ber of pounds necessary to break a given wire. The breaking stress 
varies, of course, in the same metal with the size of the wire, that 
is, with the area of its cross-section. The weight of a given wire 
varies also in the same ratio, and, therefore, in order to have a con- 
venient method for designating the breaking strength applicable 
alike to all sizes of wire of a certain grade, the breaking stress is 
frequently expressed in the number of times the weight per mile of 
the given wire necessary to break it. 

Thus, knowing that a certain grade of wire has a breaking 
strength equal to two and one-half times its weight per mile, all that 
we have to find out in order to know the breaking strength of any 
size of this same grade, is the weight per mile of that size. For 
example, a No. 12 iron wire weighs 165 pounds per mile. This we 
find out by consulting any table giving the weight of wire, or by 
weighing a known length of wire. Knowing that the breaking 
strength of this wire is 2\ times its weight per mile, we may at once 
arrive at the conclusion that the breaking strength of this particular 
size is 2.\ times 165 = 412^ pounds. 

The most important electrical property of line wire is its con- 
ductivity per unit area of cross-section. A conductor of iron may 
be made to have a resistance as low as that of a copper conductor, 
by giving it about seven times the cross-sectional area. In doing 
this, however, we make its inductive capacity greater, and, as has 
been shown, this is a disadvantage. Besides this, the greater weight 

752 



WIRE FOR TELEPHONE USE. 753 

of an iron wire of the same conductivity as that of a copper wire is 
a very objectionable feature, in that it gives the insulators and poles 
or other supports, a far greater burden than is necessary. 

The resistance of a conductor varies, of course, inversely as the 
conductivity, and therefore inversely as the cross-sectional area of 
a uniform wire. Since the weight also varies with the cross-section, 
it follows that the resistance of a wire varies inversely as its weight 
per mile. A very convenient method of comparing the relative 
resistance of various grades of metal used in making wire is to take 
as the standard of conductivity the weight per mile-ohm. The 
weight per mile-ohm of a conductor is the weight of a conductor a 
mile long, and of such uniform cross-section as to have a resistance 
of one ohm. Evidently the better the conductor, the smaller such 
a wire would be, and therefore a low value of the weight per mile- 
ohm will indicate a high conductivity. The relative conductivities 
of any two metals may be determined, knowing the weight per mile- 
ohm of each. Thus, if the weight per mile-ohm of pure copper is 
873.5 and that of a sample wire is 896, then calling the conductivity 
of pure copper 100 per cent, the conductivity of the sample will be 

- * x 100 = 97 per cent. 

In making conductivity tests, the resistance of the sample tested 
is measured, and from it is calculated the weight per mile-ohm for 
that sample. This value can then be compared with the weight 
per mile-ohm of pure copper as in the above example. By doing 
this the trouble of calculating the resistance of a pure copper wire 
of the same dimensions as that of the sample is saved. 

The diameter of wire for electrical purposes is usually expressed 
according to some gauge, and there are, unfortunately, a number of 
such. Most of the different gauges have been brought into ex- 
istence by various wire manufacturers and used in connection with 
their particular products only. In these gauges the sizes of wires 
are referred to by numbers, and in nearly every case the smaller 
numbers refer to the larger wires. A better way, and one which is 
coming into more common use, is to refer to the diameter in thou- 
sandths of an inch or in mils, as thousandths of an inch are called. 
A very convenient way of expressing the area of a wire is to give 
its cross-section in circular mils; a circular mil being the area of a 
circle, the diameter of which is one mil, or 1-1000 of an inch. This 
is better than expressing the area in square inches, because the area 
in circular mils is obtained simply by squaring the diameter of the 

48 



754 AMERICAN TELEPHONE PRACTICE. 

conductor in mils. This very simple relation between the area in 
circular mils and the diameter in mils is true, because the area of 
two circles are to each other as the square of their diameters. To 
reduce the area expressed in circular mils to square inches, multiply 
it by .7854 and divide by 1,000,000. 

It is a matter of importance, when purchasing wire in any quan- 
tity, to measure its diameter accurately, so as to be sure of obtaining 
the size ordered. It is not an uncommon thing to order a wire in 
one gauge and have your order filled in another, and the latter gauge 
usually happens to be smaller than the former. 

Circular wire gauges, such as is shown in Fig. 534, are obtain- 
able and serve their purpose well, but are subject to the disad- 




FIG. 534.-CIRCULAR WIRE GAUGE. 

vantage that a separate gauge is necessary for each particular set 
of gauge numbers. These gauges are used by inserting the wire 
into the notches in its periphery until one is found which it just 
fits; the number corresponding to that notch is then the gauge num- 
ber of the wire. A far better gauge, although one which is at first 
a little puzzling to use, is that shown in Fig. 535 and known as the 
micrometer. It consists of a yoke of tempered steel, in one side of 
which is mounted a graduated thumbscrew. The wire or other 
object to be measured is placed between the end of the thumb- 
screw and the anvil on which it rests when closed, and the screw 
turned until it makes light contact with the object on both sides. 
These screws are arranged with forty threads to the inch, so that 
one complete turn of the screw in a left-handed direction will open 



WIRE FOR TELEPHONE USE. 755 

the micrometer 1-40 of an inch. The edge of the collar carried by 
the screw is divided into twenty-five equal parts, so that a turn of 
the screw through one of these divisions will open the micrometer 
1-25 of 1-40, or 1-1000 of an inch. The shaft on which the collar 
turns is divided into tenths of an inch, and each 1-10 is subdivided 
into four parts. Thus a rotation of twenty-five divisions on the 
collar will equal one division on the shaft, or .025 inch. If the collar 
is turned so as to expose the first division on the shaft and thirteen 
divisions on itself, then the distance which the jaws have opened 
will be equal to .025 -f- .013 = .038. 

The Brown & Sharpe gauge, usually abbreviated B. & S., is prob- 
ably used more for copper wire than any other gauge, while the 
Birmingham Wire Gauge, abbreviated B. W. G., is used to a greater 
extent for iron wire. 

A decided advantage in the B. & S. gauge over any of the others 




FIG. 535.— MICROMETER. 

is that the areas of the cross-sections of the various sizes of wire 
diminish according to a geometrical progression as the gauge num- 
ber increases. The ratio in this progression is 1.26, or more accu- 
rately the cube root of two. From this it follows that when we 
have increased three sizes we have doubled the sectional area of the 
wire; and, on the other hand, when we have diminished three sizes 
we have reduced the cross-section one-half. A very convenient 
thing to remember in the B. & S. gauge in connection with copper 
wire is that the diameter of a No. 10 wire is 1-10 of an inch, and 
that the resistance per thousand feet of this wire is one ohm. These 
figures are not perfectly accurate, but enough so for most practical 
purposes. If one desires to make an approximate calculation re- 
garding the size of any wire, he may do so by remembering these 
figures, which is readily done because of the number of times the 
number ten occurs in them. For example, suppose it were desired 



756 AMERICAN TELEPHONE PRACTICE. 

to find the resistance of a No. 13 B. & S. gauge copper wire. Inas- 
much as 13 is three sizes smaller than 10, the area of a No. 13 wire 
will be one-half that of the No. 10, and its resistance per thousand 
feet double that of the No. 10, or 2 ohms. If the resistance of a 
No. 14 instead of a No. 13 were desired, it could be found by finding 
the resistance of a No. 13, as before, and multiplying by 1.26, thus 
obtaining the result 2.52 ohms. 

Table III. gives the relative sizes of various numbers of wire in 
several of the gauges which are or have been in use in this country. 

IRON WIRE. 

Iron wire corrodes so rapidly that it would be utterly useless for 
outdoor work were it not possible to protect it to some extent from 
the action of the weather. This is done by a process called gal- 
vanizing, which consists in coating wire with a thin film of metallic 
zinc. The process of manufacturing iron wire is briefly as follows: 
the iron, after being brought into the proper condition by various 
processes of rolling and purifying, is rolled into small rods, after 
which it is subjected to the process of "drawing." This process 
consists in pulling the rods through a series of dies, made of steel, 
each die being smaller than the one preceding it. This is neces- 
sarily done while the iron is cold and is termed "cold drawing." 
The successive drawings of the wire through the dies serves not 
only to reduce its cross-section, but also to render it excessively 
hard and brittle, and it is necessary, therefore, to anneal it fre- 
quently between the drawings. After the wire has been drawn to 
the proper size it is annealed and inspected and is then ready for 
galvanizing. The wire, in order to thoroughly clean its surface, is 
"pickled" in diluted sulphuric acid for a considerable length of time, 
after which it is thoroughly washed in order to remove all traces of 
acid. It is then immersed in hydrochloric acid. The wire is then 
rolled from one reel to another and between these reels it passes 
first through a furnace heated to a very high degree, immediately 
afterward through a vat containing a solution of hydrochloric acid 
which cools the wire and removes any oxides that have formed 
during the drawing, and then through a second vat containing 
molten zinc maintained at a constant temperature by a furnace 
underneath. The time between the immersion in the last acid bath 
and the zinc bath is short, because these vats are placed very 
close together, and the metal therefore has no chance to oxidize. 

As the proper galvanizing of iron wire is, all things considered, 
the most important step in its manufacture, it is very essential that 



WIRE FOR TELEPHONE USE. 



757 



TABLE III.— Table Showing Difference Between Wire Gauges in 
Decimal Parts of an Inch. 



1) 
bfl 

o 

u 

.a 

i 


a 

I. 

s 

< 




a . 
II 

m 


Mi 

2 S3 

ftp 


. 

3 « 
h 


I 

« | c 

21 C 


Brass Manufacturers 
List. 


1 


oooooo 






.46 




.464 




oooooo 


ooooo 






.43 


•45 


.432 




ooooo 


oooo 


; 4 6' * ' ' 


•454 


.393 


• 4 


•4 




0000 


ooo 


.40964 


.425 


.362 


.36 


•372 




000 


oo 


.3648 


.38 


.331 


•33 


.348 




00 


o 


•32495 


•34 


.307 


.305 


.324 







I 


.2893 


•3 


.283 


.285 


•3 




I 


2 


.25763 


.284 


.263 


.265 


.276 




2 


3 


.22942 


.259 


•244 


• 245 


.252 




3 


4 


.20431 


.238 


.225 


.225 


.232 




4 


5 


.18194 


.22 


.207 


.205 


.212 




5 


6 


.16202 


.203 


.192 


.19 


.192 




6 


7 


.14428 


.18 


.177 


.175 


.176 




7 


8 


.12849 


.165 


.162 


.16 


.16 




8 


9 


•1 1443 


.148 


.148 


.145 


.144 




9 


IO 


.10189 


.134 


.135 


•13 


.128 




10 


ii 


.090742 


.12 


.12 


• "75 


.116 




11 


12 


.080808 


.109 


.IO5 


.105 


.104 




12 


13 


.071961 


•095 


,092 


.0925 


.002 




13 


14 


.064084 


.083 


.08 


.08 


.08 .( 


^83" 


14 


15 


.057068 


.072 


.072 


.07 


.072 .< 


D72 


15 


16 


.05082 


.065 


.063 


.061 


.064 .( 


>6 5 


16 


17 


.045257 


.058 


.054 


.0525 


.056 .( 


>58 


17 


18 


.040303 


.049 


.047 


.045 


.048 .( 


349 


18 


19 


.03589 


.042 


.041 


.039 


.04 .( 


H 


19 


20 


.031961 


.035 


•035 


.034 


.036 .( 


335 


20 


21 


.028462 


.0^2 


.032 


.03 


.032 .( 


3315 


21 


22 


.025347 


.028 


.028 


.027 


.028 .( 


3295 


22 


23 


.022571 


.025 


.025 


.024 


.024 .( 


527 


23 


24 


.0201 


.022 


.023 


.0215 


.022 .< 


325 


24 


25 


.0179 


.02 


.02 


.019 


.02 .( 


323 


25 


26 


.01594 


.018 


.Ol8 


.018 


.Ol8 .( 


3205 


26 


27 


.014195 


.016 


.017 


.017 


.0164 .< 


31875 


27 


28 


.12641 


.014 


.Ol6 


.016 


.OI48 .< 


3165 


2S 


29 


.011257 


.013 


.OI5 


.015 


.OI36 .< 


3155 


29 


30 


.010025 


.012 


.OI4 


.014 


.OI24 .< 


31375 


30 


31 


.008928 


.01 


.OI35 


.013 


.OIl6 .< 


31225 


31 


32 


.00795 


.009 


.OI3 


.012 


.OI08 .< 


3H25 


32 


33 


.00708 


.008 


.Oil 


.011 


.OI .< 


DI025 


33 


34 


.006304 


.007 


.OI 


.01 


.OO92 .* 


3095 


34 


35 


.005614 


.005 


. .OO95 


.009 


.OO84 .< 


x>9 


35 


36 


.005 


.004 


.009 


.008 


.OO76 .< 


3075 


36 


37 


.004453 




.OO85 


.00725 


.OO68 


3065 


37 


38 


.003965 




.008 


.0065 


.006 .< 


30575 


3S 


39 


.003541 





.OO75 


.00575 


.OO52 .< 


305 


39 


40 


.003144 




.007 


.005 


.OO4S .( 


3045 


40 



758 AMERICAN TELEPHONE PRACTICE. 

reliable tests are made before purchasing wire for outdoor use. 
Fortunately such a test is a very easy thing to make. Several sam- 
ples of the wire should be selected at random. Each should then 
be immersed in a saturated solution of sulphate of copper for a 
period of seventy seconds. It should then be withdrawn and wiped 
clean with a cloth. This process is repeated in all four times. If, 
at the end of the fourth immersion, the wire appears black, as it did 
at the end of the first immersion, the zinc has not all been removed 
and the galvanizing may be said to have been well done ; but if the 
wire has a copper color, either as a whole or in spots, it shows that 
the zinc has been eaten away and that copper has deposited itself 
upon the iron wire. In this case the wire should be rejected. 

Iron wire which is thoroughly well galvanized is at best rather 
short-lived, and poor galvanization may result in the total loss of 
the wire within a year. Well galvanized iron wire has been known 
to last twelve years, but the conditions were very favorable. Four 
to six years probably represents a fair average for the life of wire 
of this kind, but cases are frequent where wires have been so cor- 
roded within a year as to make their replacement necessary. In 
factory districts and in railroad yards where the gases from furnaces 
come in constant contact with the wire, the life of the zinc coating 
is comparatively short. 

The grades of galvanized iron wire as used by the manufacturers 
are, if not well understood, very misleading. They are referred to 
in the following terms: Extra Best Best, Best Best, Best, and 
Steel, the first three in this list being abbreviated E. B. B., B. B., 
and B. 

Extra Best Best wire is of a very soft, high grade material, having 
the highest conductivity of all. It has sufficient tensile strength for 
all ordinary purposes, w T hile its conductivity is far superior to that 
of the other grades. It has a breaking strength of three times its 
weight per mile, and the weight per mile-ohm varies from 4500 to 
4900, 4700 being a good average. 

Best Best is less uniform and tough than the above, but is some- 
what better mechanically. It has a breaking strength of about 3.3 
times its weight per mile, and its weight per mile-ohm varies from 
5300 to 6000. 

Best should undoubtedly have been called worst, for as a rule it 
is a rather poor quality of wire, and before accepting it it should be 
very carefully tested. It is harder and less pliable than the pre- 
ceding grades, and has a weight per mile-ohm of about 6500. 



WIRE FOR TELEPHONE USE. 759 

Steel wire, which is in reality a rather low grade Bessemer process 
wire, is much stronger than any of the above grades, but is greatly 
lacking in conductivity. It has a breaking strength of about 3.7 
times its weight per mile, and its weight per mile-ohm varies be- 
tween 6000 and 7000 pounds. 

Steel wire is largely used for telephone work on very short lines, 
and if well galvanized serves its purpose admirably. In short city 
lines no difference can be noticed so far as talking results are con- 
cerned between an iron or steel and a copper circuit. The steel 
wire is, as a rule, cheaper than an Extra Best Best or the Best Best, 
and has the additional advantage of greater mechanical strength. 

The following specifications are in substance those used by the 
Western Union Telegraph Company in selecting their iron wire: 

(1) The wire shall be soft and pliable, and capable of elongating 
fifteen per cent, without breaking, after being galvanized. 

(2) Great tensile strength is not required, but the wire must not 
break under a less strain than two and one-half times its weight in 
pounds per mile. 

(3) Tests for ductility will be made as follows: Pieces of wire 
shall be gripped by two 1 vises six inches apart and twisted. The 
full number of twists must be distinctly visible between the vises on 
the six-inch piece. The number of twists in a piece six inches long 
shall not be under fifteen. 

(4) The electrical resistance of the wire in ohms per mile at a 
temperature of 68 degrees Fahrenheit must not exceed the quotient 
arising from dividing the number 4800 by the weight of the wire 
in pounds per mile. This is equivalent to saying that the weight 
per mile-ohm must not exceed 4800. The coefficient .003 will be 
allowed for each degree Fahrenheit in reducing to a standard tem- 
perature. 

(5) The wire must be well galvanized and capable of standing the 
test of dipping into sulphate of copper as stated above. 

The British Post Office Specifications require a value of the 
weight per mile-ohm of 5323. 

Table IV., taken from Roebling, gives the weight, breaking 
strength and resistance of the various sizes and grades of galvanized 
iron wire: 



760 



AMERICAN TELEPHONE PRACTICE. 
TABLE IV.— Galvanized Iron Wire. 



6 


CO 


















.s 

IH 

(LI 

£ 

s 


Weights 


Pounds. 


Breaking 
Strengths, Pounds. 


Resistance Per Mile in Ohms. 


J5 

s 

5 


1000 Feet. 


One Mile. 


Iron. 


Steel. 


E. B. B. 


B. B. 


Steel. 


o 


340 


304 


1607 


4821 


9079 


2.93 


3.42 


4-05 


I 


300 


237 


1251 


3753 


7068 


3.76 


4-4 


5-2 


2 


284 


212 


II2I 


3363 


6335 


4.19 


4.91 


5-8 


3 


25Q 


177 


932 


2796 


5268 


5.04 


5-9 


6.97 


4 


238 


149 


787 


2361 


4449 


5-97 


6.99 


8.26 


5 


220 


127 


673 


2019 


3801 


6.99 


8.18 


9.66 


6 


203 


IO9 


573 


1719 


3237 


8.21 


9.6 


11.35 


7 


l8o 


85 


45o 


1350 


2545 


10.44 


12.21 


14.43 


8 


165 


72 


378 


1134 


2138 


12.42 


14.53 


I7.I8 


9 


I48 


58 


305 


915 


1720 


15.44 


I8.06 


21.35 


IO 


134 


47 


250 


750 


1410 


18.83 


22.04 


26.04 


ii 


I20 


38 


200 


600 


1131 


23.48 


27.48 


32 47 


12 


IO9 


31 


165 


495 


933 


28.46 


33-3 


3936 


13 


95 


24 


125 


375 


709 


37.47 


43.85 


51.82 


14 


83 


18 


96 


288 


54i 


49.08 


57-44 


67.88 


15 


72 


13-7 


72 


216 


407 


65.23 


76.33 


9O.2I 


16 


65 


11. 1 


59 


177 


332 


80.03 


93.66 


I IO.7 


17 


58 


8.9 


47 


141 


264 


100.5 


120.4 


139- 


18 


49 


6.3 


33 


99 


189 


140.8 


164.8 


193.8 



COPPER WIRE. 

Copper wire is practically indestructible by exposure to ordi- 
nary climatic influences. After it is first put up it acquires a thin 
coating of oxide, and after that no change whatever takes place, 
so far as can be ascertained. The process of manufacturing cop- 
per wire is similar to that for iron wire, with the exception that 
no galvanizing is necessary. The process of drawing copper wire 
has been so greatly improved recently that the old fault, lack of 
mechanical strength, has been almost, if not quite, overcome. Cop- 
per wire is now drawn so as to possess a breaking strength of 
60,000 pounds per square inch, which is quite equal to that of some 
grades of iron wire. The difference between hard-drawn copper 
wire and soft wire is due entirely to the fact that the hard-drawn 
wire is not annealed as often between the drawings. The value of 
the weight per mile-ohm is, for good commercial wire, 882 pounds, 
the wire having a tensile strength equal to about three times its 
weight per mile. For hard-drawn wire the percentage of elon- 
gation is not nearly so high as that for iron wire, being only about 
one per cent, before breaking. 



WIRE FOR TELEPHONE USE. 



761 



The value in pounds per mile-ohm of pure annealed copper is 
859, this being based on the international ohm. 

In the following table, taken from Roebling's "Wire in Elec- 
trical Construction," the weights and resistances of the various 
B. & S. gauge numbers of copper wire are given: 

TABLE V.— Copper Wire Table. 



(A 


§ 


3 


Weights Per 


Resistance Per 1000 Feet 
in International Ohms. 


« & 


.9 












in 3 










-qO 


■£ 


U3 










i 


S 


8 


1000 Feet. 


Mil. 


At 60° F. 


At 75° F. 


z 


3 


< 










0000 


460. 


21 l60O. 


641. 


3382. 


.o48n 


.04966 


000 


410. 


I68IOO. 


509. 


2687. 


.06056 


.06251 


00 


365. 


133225. 


403. 


2129. 


.07642 


.07887 





325. 


IO5625. 


320. 


1688. 


.09639 


.09948 


1 


289. 


83521. 


253. 


1335- 


.1219 


.1258 


2 


258. 


66564. 


202. 


1064. 


.1529 


•1579 


3 


229. 


52441. 


159- 


838. 


.1941 


.2004 


4 


204. 


4l6l6. 


126. 


665. 


.2446 


.2525 


5 


182. 


33124. 


IOO. 


529- 


.3074 


.3172 


6 


162. 


26244. 


79- 


419. 


.3879 


.4004 


7 


144. 


20736. 


63. 


33i. 


.491 


.5067 


8 


128. 


16384. 


50. 


262. 


.6214 


.6413 


9 


114. 


I2996. 


39- 


208. 


.7834 


.8085 


10 


102. 


IO404. 


32. 


166. 


.9785 


I. OI 


11 


91. 


8281. 


25- 


132. 


1.229 


I.269 


12 


81. 


6561. 


20. 


105. 


1.552 


1. 601 


13 


72. 


5184. 


15.7 


83. 


1.964 


2.027 


14 


64. 


4096. 


12.4 


65. 


2.485 


2565 


15 


57- 


3249. 


9.8 


52. 


3-133 


3-234 


16 


51. 


26oi. 


7.9 


42. 


3.914 


404 


17 


45- 


2025. 


6.1 


32. 


5.028 


5.189 


18 


40. 


1600. 


4-8 


25.6 


6.363 


6.567 


19 


36. 


I296. 


3-9 


20.7 


7.855 


8.108 


20 


32. 


IO24. 


3-i 


16.4 


9.942 


10.26 


21 


38.5 


812.3 


2.5 


13. 


12.53 


12.94 


22 


25.3 


640.I 


19 


10.2 


159 


16.41 


23 


22.6 


5I0.8 


1-5 


8.2 


19.93 


20.57 


24 


20.1 


404. 


1.2 


6.5 


25.2 


26.01 


25 


27.9 


320.4 


•97 


51 


31.77 


32.79 


26 


159 


252.8 


•77 


4- 


40.27 


41.56 


27 


14.2 


201.6 


.61 


3-2 


50.49 


52.11 


28 


12.6 


158.8 


.48 


2.5 


64.13 


66.18 


29 


1 1.3 


127.7 


•39 


2. 


79-73 


82.29 


30 


10. 


IOO. 


•3 


1.6 


101.8 


105. 1 


3i 


18.9 


79-2 


.24 


1.27 


128.5 


132.7 


32 


8. 


64. 


.19 


1.02 


151. 1 


164.2 


33 


7-1 


50.4 


•15 


.81 


202. 


208.4 


34 


6.3 


39-7 


.12 


.63 


256.5 


264.7 


35 


5.6 


31.4 


.095 


•5 


324.6 


335- 1 


36 


5- 


25. 


.076 


•4 


407.2 


420.3 



762 AMERICAN TELEPHONE PRACTICE. 

Abbott gives the following specifications, which contain much 
information governing the requirements to be made of manufac- 
turers in purchasing copper wire: 

COPPER WIRE. 

1. Finish. — Each coil shall be drawn in one length and be ex- 
empt from joints or splices. All wire shall be truly cylindrical 
and fully up to gauge specified for each size, and must not contain 
any scale, inequalities, flaws, cold shuts, seams, or other imper- 
fections. 

2. Inspection. — The purchaser will appoint an inspector, who 
shall be supplied by the manufacturer with all facilities which 
may be required for examining the finished product or any of 
the processes of manufacture. The inspector shall have the priv- 
ilege of overseeing the packing and shipping of the samples. The 
inspector will reject any and all wire which does not fully come 
up to all the specification requirements. The purchaser further 
reserves the right to reject on reception any or all lots of wire 
which do not fulfill the specifications, even though they shall 
previously have been passed or accepted by the inspector. 

3. Apparatus. — The manufacturer must supply at the mill the 
necessary apparatus for making the examination called for. This 
apparatus shall consist of a tension-testing machine, a torsion-test- 
ing machine, an elongation gauge, an accurate platform scale, and 
an accurate bridge and battery. Each of these pieces of appa- 
ratus may be examined by, and shall be satisfactory, to the in- 
spector. 

4. Packing for Shipment. — When ready for shipment each coil 
must be securely tied with not less than four separate pieces of 
strong twine and shall be protected by a sufficient wrapping of 
burlap so the wire may not be injured during transportation. 
The wrappings shall be placed upon the wire bundles, after they 
have been coiled and secured by the twine. The diameter of the 
eye of each coil shall be prescribed by the inspector, and all coils 
shipped shall not vary more than two inches in the diameter of the 
eye. 

5. Weight. — Each coil shall have its length and weight plainly 
and indelibly marked upon two brass tags, which shall be secured 
to the coil, one inside the wrapping and the other outside. 

6. Mechanical Properties. — All wire shall be fully and truly up 
to gauge standard, as per B. & S. wire gauge. The wire shall 



WIRE FOR TELEPHONE USE. 



be cylindrical in every respect. The inspector shall test the size 
and roundness of the wire by measuring both ends of each coil, 
and also by measuring at least four places in the length of each 
coil. A variation of not more than ij mil on either side of the 
specified wire-gauge number will be allowed, and the wire must 
be truly round within one mil upon opposite diameters at the same 
point of measurement. The strength of the wire shall be deter- 
mined by taking a sample from one end of each coil, 30" in length. 
Of this piece 18" shall be tested for tension and elongation by 
breaking the same in the tension-testing machine. The samples 
should show a strength in accordance with the following table: 



TABLE VI. 



■Breaking Weight of Hard-Drawn and Annealed Copper 
Wire. 



Size of Wire, B. & S. Gauge. 


Breaking Weight of Hard- 
Drawn — Pounds. 


Breaking Weight of Annealed — 
Pounds. 


OOOO 


9971 


5650 


OOO 


7907 


4480 


OO 


627I 


3553 


O 


4973 


2818 


I 


3943 


2234 


2 


3127 


1772 


3 


2480 


1405 


4 


1967 


1114 


5 


1559 


883 


6 


1237 


700 


7 


980 


555 


8 


778 


440 


9 


617 


349 


10 


489 


277 


11 


388 


219 


12 


307 


174 


13 


244 


138 


14 


193 


109 


15 


153 


87 


16 


133 


69 


17 


97 


55 


18 


77 


43 


19 


61 


34 


20 


48 


27 



A variation of i^ per cent, on either side of the tabular limits 
will be accepted by the inspector. The elongation of the wire 
must be at least three per cent, for all sizes larger than No. 1 ; 
ij per cent, from No. 1 to No. 10, and 1 per cent, for sizes less 
than No. 10, for hard-drawn copper wire. The remainder of the 
sample selected will be tested for torsion. The torsion sample 
will be twisted in the torsion-testinq- machine to destruction, one 



764 



AMERICAN TELEPHONE PRACTICE. 



foot in length being placed between the jaws of the machine. 
Under these circumstances hard-drawn copper wire shall show 
not less than 20 twists for sizes over Xo. 1 ; from 40 to 90 twists 
in sizes from Xo. 1 to Xo. 10; and not less than 100 twists in 
sizes less than Xo. 10. Should the sample selected from one end 
of each coil show failure to come up to the specifications, the 
inspector may take a second sample from the other end of the 
coil. If the average of the results from both samples shall be 
within the specifications, the coil shall be accepted; if not within the 
specifications, the coil shall be rejected. The weight per mile shall 
be determined by carefully weighing 2 per cent, of the number of 
coils called for in the contract, and the weight thus obtained shall 
correspond, within 2 per cent., on either side of the result given in 
the following formulae. 

CM 

Weight per mile = — : — • 

62.567 

«r ■ 1. ' CM 

Weight per 1000 it. =- 



330-353 



7. Electrical Properties. — The electrical properties of the wire 
shall be determined by the inspector selecting 3 per cent, of the 
coils, and from them taking lengths of 100 ft., 500 ft., or tooo ft., 
at his discretion, and measuring the conductivity of the same with 
a standard bridge. For soft-drawn copper wire the following re- 
sistance per mil-foot will be assumed: 

TABLE VII.— Resistance of Copper Wire at Various Temperatures. 



Temperature in 


Resistance, Legal 


Temperature in 


Resistance. Legal 


Degrees F. 


Ohms. 


Degrees F. 


Ohms. 


O 


8.96707 


60 


IO.20253 


IO 


9.16413 


70 


10.42083 


20 


9-36473 


80 


IO.64268 


30 


9.56887 


90 


IO.86806 


40 


9-77655 


IOO 


II.09698 


50 


9.98777 







For hard-drawn wire the resistance per mil-foot shall be 1.0226 
times the foregoing figures. All wire shall be within 2 per cent, 
of the above figures. 

So far in this chapter only bare wires such as are used in out- 
side construction work have been considered. The subject of in- 
sulated wire is no less important, but is more difficult to treat of 



WIRE FOR TELEPHONE USE. 765 

comprehensively owing to the great variety of such wires as well 
as the great number of uses to which they are put. Wire having 
a single, double or triple wrapping of either cotton or silk, unim- 
pregnated with insulating compound, may be classified under the 
general heading of magnet wire, as it is used almost exclusively for 
the winding of magnets. In telephone work, single silk magnet 
wire, by which is meant a copper wire wrapped with a single layer 
of silk, is by far the most common, and for most purposes by far 
the best. Silk, even in cases where high insulation is not of great 
importance, makes a much more economical insulation in telephone 
magnet work than cotton, because, on account of the extremely 
thin insulation afTorded by silk a much greater number of turns may 
be put on a given spool with the same length of wire than when cot- 
ton insulation is used. 

The thickness of the insulation of wire is a point concerning 
which even manufacturers of telephone apparatus have been negli- 
gent as to their own interests. The difference between the cost of 
winding a given magnet with a given number of turns with fine wire 
(say No. 36) with a 3-mil insulation and with a i-J-mil insulation is 
astonishing. Not only is' the amount of silk required per pound 
of wire much less with the thinner insulation, but the length of wire 
required to secure the requisite number of turns is also greatly re- 
duced when the thinner insulation is used, on account of the smaller 
diameter assumed by the coil. 

Manufacturers of thin insulated wire are experiencing some 
trouble in securing a perfectly uniform covering of a thickness of 
only i\ mil, but it can be done at a somewhat increased expense for 
labor. The consumer, however, would convince himself with a little 
thought and experimenting that the thinnest covering which affords 
ample protection, so far as insulation is concerned, is cheaper even 
when the wire costs a considerably greater price per pound. 

There is a class of wire commonly known as office wire, which is 
used largely for the interior wiring of houses for such work as the 
installation of electric door bells, annunciators, etc., and was at one 
time largely used in telephone work. This consists of a copper 
wire insulated with one wrapping of cotton in one direction and 
another wrapping of cotton in another direction, the whole being- 
saturated with paraffin. This wire is unsatisfactory from many 
points; it is not moisture proof, its insulation is not high, and. fur- 
thermore, it is highly inflammable. 

For interior wiring now in telephone work a wire which is heavily 



766 AMERICAN TELEPHOXE PRACTICE. 

coated with a good grade of rubber and afterwards braided over 
with cotton is being used to a greater and greater extent. For out- 
side construction, such as is required in running drop wires from 
telephone poles to subscribers' premises and similar work, the best 
possible grade of rubber-covered wire is rapidly supplanting the 
cheaper grades, and the old so-called weather-proof wire containing 
no rubber is almost a thing of the past for this work. 

The following specifications for No. 18 rubber-covered wire have 
been used by the engineers of one of the large independent operating 
companies: 

RUBBER-COVERED TELEPHONE WIRE 
(NO. l8 B. & S. G.). 

wire : 

The wire to be furnished under these specifications is No. 18 B. 
& S. G. rubber-covered copper wire. 

The copper shall be soft drawn and shall have a conductivity of 
not less than 98 per cent. 

The wire shall be perfectly symmetrical, uniform in quality, 
pliable, free from scales, inequalities, flaws, splits, and other defects. 
kind: 

There are four different kinds of wire to be furnished. 

First. Plain rubber insulated wire, twisted in pairs. 

Second. Twisted pairs, of which each wire is insulated with 
rubber and then the braid woven on; the two separately braided 
wires then being twisted. 

Third. Single wire, plain rubber covered. 

Fourth. Single wire, rubber covered and then braided. 

insulation: 

The rubber compound shall be of the highest grade and of such 
composition as to be thoroughly waterproof and insure a perma- 
nently tough and lasting form of insulation. 

The insulation must show a resistance of one megohm per mile 
after two weeks' submersion in water at 70 degrees Fahrenheit, and 
three days' submersion in lime water, and after three minutes' elec- 
trification at 550 volts. 

The thickness of the rubber insulation shall be such that the 
diameter of the finished wire shall not be less than 3-32 of an inch 
under the braiding. 

The insulation shall be applied to the copper wire so that it will 
be concentric with the wire, and of equal thickness at all places. 



WIRE FOR TELEPHONE USE. 767 

For the braided wire a close braid of cotton shall be put on cov- 
ering the wire, and in one pair of the twisted, braided wire, a 
differently colored thread, either green or red, shall be woven in the 
braid, so that the wires of the pair shall be distinctly marked one 
from the other. 

The color of the braiding shall be brown of an approved shade, to 
conform with samples submitted with the proposal. 
coils: 

The wire shall be put up in coils, which shall contain continuous 
lengths of not less than 2000 feet, in which no joints or splices of 
any kind will be allowed. 



CHAPTER XXXIX. 
POLE LINE CONSTRUCTION. 

The poles most used in the United States are of Norway pine, 
chestnut, cedar -and cypress. Southern pine is not as durable as 
Northern pine, although it is used to a large extent in the South. 
Canadian or Michigan cedar is, however, all things considered, the 
best wood to use. 

The average life of the various woods mentioned are, according 
to Maver, as follows: 

Norway pine 6 years 

Chestnut 15 

Cedar 12 " 

Cypress 10 " 

In choosing the kind of pole to be used, the locality must always 
be considered, for obviously it might be poor economy to bring 
cedar poles from Michigan for the reason that they would last per- 
haps a few more years than cypress poles, which would be cut on the 
ground. 

Poles should be well seasoned before setting in the ground. This 
is accomplished by natural process of drying. Before seasoning, 
however, the pole should be peeled and all knots trimmed. It is 
easier to do this while the sap is in them than afterwards, and, more- 
over, the drying takes place in a shorter time if the bark is removed. 
If the pole is not seasoned before setting or before it is painted, where 
it is to be painted, the sap is almost sure to cause a dry rot, which 
will eventually destroy the pole. The worst feature of this trouble 
is that the defect is not noticeable on the surface and therefore is 
likely to cause trouble when least expected. A pole may have all 
external appearances of being perfectly sound and yet be a mere 
shell, so that, when subjected to some heavy storm it goes down, 
perhaps carrying many other poles with it. 

Practice differs to some extent concerning the size of poles. 
Money saved, however, in the purchase of light poles is usually 
saved at a great cost in the future. Table VIII gives a list of the 
sizes which in most cases meet the demands of the best practice to- 

768 



POLE LINE CONSTRUCTION. 



r69 



day. There is now a growing tendency to use in the best work, 
even heavier poles than these. 

TABLE VIII. 



Length. 


Diameter at 
Top. 


Diameter 6 ft. 
from Butt. 


Length. 


Diameter at 
Top. 


Diameter 6 ft. 
from Butt. 


25 feet. 
30 " 
35 " 
40 " 

45 " 


7 inches. 
7 " 
7 " 
7 " 
7 " 


9 inches. 

10 " 

11 " 

12 " 

13 " 


50 feet. 

55 " 
60 " 

65 " 
70 '< 


7 inches. 

7 " 
7 " 
7 " 
7 " 


14 inches. 

16 " 

17 " 

18 " 
20 " 



Telephone companies that have been in the field long enough 
have learned that the days of "fence-post" construction are over, 
and that in the long run poor construction is much more expensive 
than good. To be sure, in many of the Independent installations 
it is a matter of necessity to use a medium construction throughout 
on account of the first expense, and in such case if the dimensions 
of the poles given in the table above are too expensive, they will at 
least serve as a standard at which to aim. 

The number of wires to be carried on any pole line is also a 
question that will largely determine the diameter of the poles. On 
the corners, or where a heavy lead is dead-ended, to make connec- 
tion perhaps with an underground cable, the poles used should be 
in many cases much larger than those given. In fact, in such cases 
the heaviest poles that can be had will be none too large. 

The question of the number of poles to the mile is one that must 
be decided to meet the particular conditions of the line to be erected. 
The greater the number of poles the lower the insulation, but this 
is of no importance, and is more than offset by the greater freedom 
from breakage of wires and consequent decrease in the expense of 
maintenance when the poles are set closely together. In this conn- 
try on long-distance lines the best practice dictates the use of from 
forty to fifty to the mile, although many lines are operated with 
thirty or less. As a rule, the greater the number of wires carried. 
the closer and heavier the poles should be. The liability of any 
particular locality to heavy sleet and wind storms is another factor 
in determining the size and distribution of poles. In the long-dis- 
tance lines of the American Telegraph and Telephone Company the 
standard distance between the poles is 130 feet, making approxi- 
mately forty to the mile. 



770 



AMERICAN TELEPHONE PRACTICE. 



In the past the standard has been, on long-distance lines, to use 
no pole shorter than 35 feet, many of course being necessarily much 
longer. Recent practice, however, seems to tend toward shorter 
poles, bringing the wires closer to the ground. 

In cities poles varying from 30 to 60 feet are, as a rule, used. 
These are generally of cedar. It is frequently necessary to use a 




FIG. 536.— POLE STRIPS AND BUTT PLATES. 



longer pole in city work in order that the line may be carried above 
electric light and power circuits. 

It is often necessary to protect poles along the streets of towns 
and cities from the gnawing of horses hitched to them, and also 
from the wearing effects of wagon-hubs, which often greatly weaken 
the poles at a point where they are least able to stand it. Galvan- 



POLE LINE CONSTRUCTION. 



Ill 



ized steel protecting strips are obtainable for the former purpose, 
and what are termed butt-plates, about 15 inches by 18 inches by 
3-16 inch thick of the same material, ma'y also be purchased from 
supply dealers for the latter purpose. A pole thus equipped is 
shown in Fig. 536. Attention has been recently called to the fact 
that butt-plates sometimes cause rotting of the pole by holding 
moisture. For this reason, ^-inch strap iron, or, better still, regular 
26-inch cross-arm braces fastened on with small lag screws and 
spaced about ij inches apart, seems better practice. 

In Table IX. is given some useful information concerning the 
weights of poles of various sizes and the number to a single or 
double carload: 

TABLE IX.— Cedar Poles. 



Single 


Cars. 




Number in 


Load. 




4 inches, 


25 feet. 


Not less than 


175 


and up 


to 225 


5 " 


25 " 


«< 


" 


I50 


<< 


1 200 


6 " 


25 " 




" 


IOO 


" 


1 125 


7 " 


25 " 






75 




' IOO 


6 " 


30 " 




" 


75 




' IOO 


7 " 


30 " 


«( 


«< 


60 


<< 


« 80 


7 " 


35 '\ 


(< 




55 




' 75 


Double Cars. 




Number in 


Load. 




7 inches, 40 feet. 


Not less than 60 and up 


to 75 


7 " 


45 " 




" 


5o 


(< 


* 65 


7 " 


50 " 






40 


<< i 


' 5o 


7 " 


55 " 






35 


a 


4 45 


7 " 


60 " 




" 


25 


" ' 


1 35 


7 " 


65 " 






20 


<< < 


' 25 



4-inch top, 25 feet. 



25 
25 
25 

30 
30 
35 
40 

45 
50 
55 
60 
65 



Weights. 



Green. 


Seasoned. 


200 pounds 


155 pounds 


260 " 


200 u 


325 " 


250 " 


425 " 


350 " 


425 " 


35o " 


500 


450 


750 " 


650 


1,075 " 


850 " 


1,150 " 


1,000 " 


1,400 " 


1,250 " 


1.875 M 


1.650 u 


2,300 *' 


3.000 " 


2,800 " 


2,500 



AMERICAN TELEPHONE PRACTICE. 



Table X gives similar data regarding Norway pine poles. 
TABLE X.— Norway Pine. 







Weight 


Number 






Weigh i 


Xumber 


Length. 


Top. 


in 


in 


Length. 


Top. 


in 


in 






Pounds. 


Load. 






Pounds. 


Load. 


40 feet 


7 inches 


I,IOO 


90 


65 feet 


7 inches 


2,000 


45 


45 " 


7 " 


1,200 


So 


70 •« 


7 " 


2,400 


50 


50 " 


7 " 


1,350 


72 


75 " 


7 " 


2,800 


45 


55 " 


7 " 


1,500 


t>5 


fco " 


7 " 


3,400 


35 


Co •« 


7 " 


1,700 


55 


85 " 


7 " 


3,800 


30 



Twenty-five and 30-foot poles should be loaded on cars taking a 
minimum of 24,000 pounds ; 35-foot poles on cars taking a minimum 
of 30,000 pounds; double loads (40 feet and longer), on two cars 
30,000 each or 60,000 minimum for the double load. 

All poles up to and including 7 inches — 35 feet, will be loaded 
on single cars. 

It is not customary in this country to treat poles with any pre- 
serving process, but it is always well to coat the pole for a distance 
of six feet from the butt with pitch before setting it. It is also w r ell 
to give city poles two coats of good oil paint, and a very neat 
appearance is added if the lower portions are painted black or daik 
green to a distance of six feet above the ground, while the remain- 
ing portion is painted some light color. 

In some quarters a process termed creosoting is meeting with 
favor for preserving telephone and telegraph poles. It is probably 
the cheapest of all effectual processes of this kind, and consists, 
briefly, in placing the pole in an air-tight cylindrical iron chamber, 
after which steam, at a pressure of about 100 pounds to the square 
inch, is admitted and the poles are subjected to this treatment for 
about four hours. This vaporizes the sap and wood acids, and 
completely sterilizes the poles throughout, killing all germs which 
might afterwards cause fungus and decay. When this is accom- 
plished the steam is released and the chamber subjected to vacuum, 
by which all moisture and organic matter is extracted and the wood 
left in a porous condition. While in vacuum the chamber is kept 
hot by steam coils, thus aiding in the drying. The poles, may now 
be called antiseptic, and are ready for the preserving fluid. After 
this dead oil of coal tar is forced into the cylinder under a heavy 
pressure, and it is found that it penetrates to the very heart of the 
poles, thus adding very materially to their lasting qualities. Cases 



POLE LINE CONSTRUCTION. 



73 



are cited where poles treated by this method have been perfectly 
sound after having been in service for a period of twenty years. 

A cheaper process involves the sterilization of the pole as above 
stated and then injecting into its pores a solution of chloride of zinc. 
This is sometimes followed by a treatment of dead oil of coal tar 
in order to prevent the subsequent dissolving of the chloride in 
moist climates. 

Another process, termed vulcanizing, consists in heating the pole 
in a closed vessel for several hours to a temperature of about 500 F. 
The principle in this treatment is that the intense heat causes the 
sap in the wood to coagulate, after which it can produce no evil 




FIG. 537.-CROSS ARM. 

effects. This would apparently be cheaper than creosoting, or the 
chloride of zinc process. 

Several forms of structural iron or steel poles have recently been 
put on the market, for which are claimed low depreciation and low 
cost of maintenance as well as great strength and sightliness. They 
have not as yet been widely adopted. 

The cross-arms carrying the insulators are preferably of sawed 
yellow pine, as shown in Fig. 537. The size in general use is 4J" 
by 3i" '• The lengths vary from 3 to 10 feet, according to the num- 
ber of pins or insulators to be used. Table XI shows the lengths of 
the various standard cross-arms; also the spacings of the pin-holes. 



TABLE XT. 









Spacings. 


Length. 


Number or 
Pins. 


End. 
















Center. 


Sides. 


3 feet 


2 


4 inches 


28 inches 




4 " 


4 


4 " 


16 " 


12 inches 


5 " 


4 


4 " 


18 " 


17 " 


6 " 


4 


4 " 


22 " 


21 " 


6 " 


6 


4 " 


16 " 


12 " 


8 " 


6 


4 " 


18 " 


I7# " 


8 " 


8 


4 " 


16 " 


12 " 


10 " 


8 


4 " 


17* " 


15* •' 


10 " 


10 


4 " 


16 " 


12 " 



74 



AMERICAN TELEPHONE PRACTICE. 



The standard size of pin, Fig. 538, for the above arm has a i-J-inch 
shank, and arms of this size are usually bored accordingly. They 
are also bored, as shown in Fig. 537, with two -J-inch holes for lag- 
screws used in attaching them to the poles. 

Another size of cross-arm, called the telephone arm, once came 
into use to a considerable extent for cheaper installation, but has 



FIG. 538.— INSULATOR PIN. 

now been generally abandoned in good construction. The size of 
this arm is 2$ by 3J , being ^ inch smaller in each dimension than 
the standard. These arms are usually bored for i4;-inch pins and 
the length of a ten-pin arm is only 8J feet. 

All cross-arms should be given two coats of good metallic paint, 
usually red, before setting in position. In order to attach them to 
the pole a gain is cut in the pole of such dimensions as to accurately 
fit the longest side of the cross-arm. The gain, as a rule, should 




FIG. 539.— LAG SCREW. 



not be more than one-half inch deep, however, for the reason that 
a greater depth is likely to weaken the pole unduly. The gain 
should be given two coats of good white lead before the cross-arm 
is put in place. The common way of attaching the cross-arms to 
the pole is by two lag-screws of the type shown in Fig. 539. These 
are of such length as to reach almost through the pole, and their 
threads are cut in such a manner that they may be driven part of 




FIG. 540.— CARRIAGE BOLT. 



the way home. A much better practice now is to attach the cross- 
arm to the pole by means of a single carriage bolt, or "through 
bolt," Fig. 540, extending entirely through the arm and pole, being 
secured by a nut and a washer. This method has an advantage 
over the use of lag-screws in point of strength and durability. The 
hole for the carriage bolt may be bored perfectly smooth and clean, 



POLE LINE CONSTRUCTION. 775 

and of such size as to accurately fit the carriage bolt, so there is 
little chance for rotting. Another way, not easy to follow on ac- 
count of the varying sizes of pole tops, is to bore no hole whatever 
through the pole, but to attach the cross-arm by means of a U-bolt 
extending through the cross-arm and around the pole and secured 
from the front by means of two nuts. This is little used. 




FIG. 541.— REINFORCED CROSS-ARM BRACE. 



The arm is further braced in any case by the use of wrought-iron 
or steel strips, commonly termed cross-arm braces. These usually 
consist of straight flat bars not smaller than i\ inch wide by \ inch 
thick, and varying in length from 20 to 30 inches. A hole is 
usually punched in one end for the reception of a ^-inch lag-screw 
and in the other for a f-inch carriage bolt. The two braces for 
each cross-arm are attached by single lag-screws to the pole at a 
distance varying from 16 to 18 inches from the bottom of the* arm. 




METHOD OF ATTACHING BRACES. 



The other ends of the braces are attached by carriage bolts to the 
cross-arms at points about equal distances from the pole. 

A new cross-arm brace has recently been produced, this being 
formed up of sheet steel with reinforced edges. The reinforcements 
are formed by rolling the metal into hollow cylinders along the 
edges of the brace. This seems meritorious in that it is lighter than 



776 AMERICAX TELEPHONE PRACTICE. 

the regular solid brace and is said to be stronger and cheaper. It 
would, apparently, however, be more liable to rust through. This 
brace and the usual method of using braces are shown in Figs. 541 
and 542, respectively. 

In all cases suitable washers should be used under carriage 
bolt nuts and heads, and under lag-screw heads where they are used 
in attaching an arm to the pole. All hardware to be used on poles, 
such as bolts, washers, braces, etc., should be thoroughly galvanized 
and should be made to stand the same test that is required on gal- 
vanized iron wire — that is, four successive plunges of seventy sec- 
onds each in a saturated solution of sulphate of copper without re- 
moving all of the zinc coating. 

The pins most commonly used are of locust or of oak. The 





FIG. 543.— ROOFING AND GAINING. 



former is by far the better, as it is stronger and more capable of re- 
sisting the action of the weather. It is, however, nearly twice as 
expensive as oak. The pins should be turned from split wood in 
order that they may not be cross-grained, and all pins should be 
given two coats of the same kind of paint that is used on cross- 
arms, or dipped in white lead just before driving them into the 
cross-arm. They should be nailed in the cross-arm with a sixpenny 
nail. 

In some cases on corners, or in places where excessively heavy 
strain will be brought upon a pin, it is advisable to use wrought- 
iron or steel pins, but these must be used with caution, as in many 
cases they have proven inferior to wooden pins, being so soft that 
they bend to a horizontal position when subjected to the strain. 



POLE LINE CONSTRUCTION. 777 

The top of the pole should be roofed as shown in Fig. 543. The 
center of the first gain should be about 10 inches below the peak 
of the roof. The spacing between cross-arms depends on circum- 
stances. For city work 18 or 20 inches between centers is good 
practice. For toll line work this spacing should not be less than 
20 inches, and in some cases the American Telephone and Tele- 
graph Company use 24 inches. 

The insulators used in this country are universally made of glass. 
Blown glass has been found to be much superior in insulating quali- 
ties to molded glass, but the latter is so very much cheaper that it 
is always furnished. Fig. 544 shows a form of insulator largely 
used in telephone work, called the "pony" insulator. There is 
another style, termed the "double-petticoat" insulator. It is so 
termed from the fact that it has two downwardly projecting flanges 




FIG. 544.— STANDARD PONY INSULATOR. 

or "petticoats," the idea of this being that the path for leakage from 
the line to the pin is thereby rendered considerably longer, the leak- 
age current having to pass up and down the surfaces of both petti- 
coats in series. 

For making transpositions a double glass insulator. Fig. 545, is 
used. This requires a longer pin than the standard. 

In an interesting series of experiments described by Abbott, it was 
found that the insulating quality of glass insulators varied largely 
with the condition of the surface of the insulator. These experi- 
ments were conducted over a period of one hundred and fifty days. 
observations being made once a day. The general result indicated 
that the greatest loss in insulation occurred during foggy or misty 
weather. During heavy rainstorms the insulation was somewhat 
higher and after the storm, when the insulators had been dried, 
the resistance of the line was considerably higher, owing to the 



778 



AMERICAN TELEPHONE PRACTICE. 



cleaner condition of the surface. In good weather the double-petti- 
coat insulators gave much higher resistance than the single or cor- 
responding size, but during a rainstorm a double-petticoat form was 
inferior to the single, although it was found to dry more rapidly after 
a storm. 

The determination of the pole line route is a matter of no small 
importance. Right of way must be secured, and this usually calls 
forth all the ingenuity of the party unfortunate enough to be assigned 
to that duty. Before distributing the poles and other material the 
route should be thoroughly studied in every detail. Stakes should 
be driven marking the location of the poles. It should be borne in 
mind in locating these stakes that bends in the pole line should be 
avoided wherever possible, that the ground should be of such nature 
as to form as good a support as possible for the pole, that there shall 
be no interference from trees, houses or other poles, and lastly 




FIG. 545.— DOUBLE-GLASS TRANSPOSITION INSULATOR. 



that the route shall be as direct as possible. When a turn must be 
made it should be so located, if possible, that the guy wire required 
to hold up the corner will have suitable anchoring ground. Lack 
of attention to these preliminary details too often brings an endless 
amount of trouble in the way of rehandling of poles, redigging of 
holes and similar useless labor. 

When the ground is level or gently undulating, no provision need 
be made for grading the pole tops. Where, however, the country 
is hilly it is well to make a survey of the route with a level, plac- 
ing the instrument between each successive pair of stakes and taking 
a front and back sight from each position to the adjacent stakes. 
A record of the data thus obtained will enable one to plat the ver- 
tical section of the route. The profile of the pole tops may then be 



POLE LINE CONSTRUCTION. 



779 



platted, care being taken to smooth out all sharp bends in it. This 
is accomplished by putting the tallest poles in the hollows and the 
shortest on the hilltops. The same results may be accomplished, 
though not so well, without the use of the level, but it requires an 
experienced eye to do it to best advantage. 

After having decided on the location of the poles, the length of 




FIG. 546.— CARRY HOOK. 

pole for each point and all other preliminary details, such as placing 
of heavy poles at the corners, the poles may be hauled and distrib- 
uted along the route. They should be laid with the butt near the 
stakes and pointing down hill if on a grade. 

The poles are distributed along the route by any available means. 
If the line runs along the railroad, they may be rolled from the 
flat car at the proper intervals and carried to their places by carry- 
ing hooks (Fig. 546). If the line is a long one and does not follow 
the line of a railroad, the poles should be unloaded from the cars at 
convenient points, and hauled to their proper locations by wagons. 

Poles of medium length may, under ordinary circumstances, be 
raised with the cross-arms in place, and as they are much more 




FIG. 547.— SHOVEL. 



easily attached on the ground, this should always be done where 
possible. Where this is done the arms should not be permanently 
secured in place as they need to be levelled after the pole is set. 
Where the arms are not in place when the pole is raised it is con- 
venient to nail a lath in the gain, in order that the proper alignment 
of the cross-arms may be secured. 

In digging the pole holes long-handled shovels (Fig. 547^ and 



780 



AMERICAN TELEPHONE PRACTICE. 



spoon shovels (Fig. 548), having 7 and 8 foot handles, are used 
in conjunction with 8-foot steel digging bars, shown in Fig. 549. 
Sometimes the post-hole auger (Fig. 550) is used, but this is only 
where the conditions are very favorable. Dynamite, judiciously 




FIG. 548.— SPOON SHOVEL. 

applied, is now being used successfully in digging holes, even where 
the soil is of such a nature as not to absolutely require its use. 

No definite rule can be given for the depth at which poles should 
be set in the ground. The character of the soil, the distance between 
poles, the number of wires carried, the height of the pole and the 
sharpness of the turns made in the line must all be considered in 



FIG. 549.-DIGGING BAR. 



determining this question. For average work the data given in 
Table XII are believed to be in accordance with the best practice. 

table xii. 

25-foot pole, 5 feet in ground 50-foot pole, 7 feet in ground 

30 " "5 " " 55 " " 7V2 " 

35 " " 5 J A " " 60 " " 8 " 

40 " " 6 " " 65 " " 8 " 

45 " " 6H " " 70 " " 8H " 



§b 



FIG. 550.— AUGER. 




On curves or corners the holes should be dug from 6 inches to a 
foot deeper than is specified in this table. 

After digging the holes the poles are carried or rolled by cant- 
hooks (Fig. 551) so that the butt of each is over its hole. A 
piece of scantling, or preferably a hardwood board, is placed in the 



POLE LINE CONSTRUCTION. 



"81 



hole to serve as a rest for the butt of the pole while it is being 
raised. The use of this butt-board prevents the crumbling of the 
earth which is sure to result and cause much trouble if this precau- 
tion is not taken. 

The tools required in raising poles of the average length — from 



fl a s c 



r, !-,-,,v 



= 




FIG. 551.— CANT HOOK. 



30 or 50 feet — are five or six pike-poles (Fig. 552), ranging from 
12 to 16 feet in length, and two dead men, or pole supports, of which 
two types are shown in Figs. 553 and 554. 

The pole is raised slightly and its end slipped into the hole resting 



FIG! 552.— PIKE-POLE. 



all the while against the butt-board or scantling. The small end 
of the pole is then raised higher and the pole support placed under it, 
while the men obtain another hold. The pole is raised gradually, 
the support being each time moved closer to the butt. When too 




FIG. 553.— JENNY POLE SUPPORT. 



high to be handled directly, the pike-poles are used on its upper part 
(Figs. 556 and 557), and in this way it is readily raised into a vor- 
tical position, slipping into the hole while bearing against the butt- 
board. It is then braced by the pike-poles, as shown in Fig. 558, 



782 



AMERICAN TELEPHONE PRACTICE. 



and turned by means of cant-hooks, so that the lath or cross-arms, 
if they were attached before raising the pole, are in proper posi- 
tion ; it being remembered that the cross-arms should face each other 
on every alternate pair of poles. The hole is then filled in with the 
soil which was removed from it in digging, the soil being thoroughly 
tamped with tamping bars, shown in Fig. 555, from the bottom up. 
Great care should be taken that the shoveling in is not done so fast 
that the earth can not be properly tamped. This is frequently the 




FIG. 554.— MULE POLE SUPPORT. 



cause of much trouble, and, while it expedites the erecting of the 
poles, it causes much loss of time and money later, on account of the 
poles giving way when placed under strain. 

A later and much more economical way of setting poles where 
large numbers are to be set, is by the use of a pole derrick wagon, 
which consists merely of a heavy wagon with low platform, upon 
the rear of which is placed a boom to be used in raising the pole. 
This boom is made adjustable with respect to the wagon, and is pro- 
vided with guys extending from the forward portion of the truck. 
A block and tackle supported from the end of the boom serves as a 



2 X Yi IRON. 




FIG. 



-TAMPING BAR. 



means for raising the pole, the necessary power being applied by 
horses operating through the intervention of a snatch-block. Figs. 
559, 560 and 561 show such a derrick wagon, these cuts being made 
from photographs loaned by W. H. Anderson & Sons, of Detroit, 
Mich., who manufacture the wagon. 

As will be seen, the platform of the wagon is very low, being sup- 
ported beneath the axles. The boom is mounted on a trunion at 
the rear of the wagon, and is made of a piece of 8 inch steel tubing 



POLE LINE CONSTRUCTION. 



783 



,■ ■ 



& 








FIG. 556.— RAISING POLE. 



i- ~w 




k- 



■:> 



• ».> : - 






r, • 



''Jf* 11 ' 



.^»^ 



^W 







jAMBJLBLEC: 



FIG. 557. R MSTNG POT.TC. 



784 



AMERICAN TELEPHONE PRACTICE. 



iy feet in length into which a hardwood spar is inserted, so that a 
telescopic adjustment is afforded, thus giving a long boom suitable to 
raising different lengths of poles. By this means. poles 70 feet in 
length are easily handled. On the front end of the wagon is 
mounted a hand-operated hoisting winch for adjusting the guys 
leading to the end of the boom. By this means the angular position 
of the boom may be adjusted to any degree even while subjected to 
the weight of the pole. 

Steam or electric power has been proposed for raising the poles 





FIG. 55S.— POLE RAISED. 



when derrick wagons are used, but this is not thought to be econom- 
ical, for the presence of horses is needed in any event to draw the 
wagon to the place of work and from pole to pole thereafter. With 
the wagon shown it is not necessary for the team to be hitched to 
the wagon tongue to move from pole to pole for arrangements are 
made whereby the team is hitched to the rear end of the wagon, 
and one of the crew, taking hold of the tongue, steers the wagon to 
its proper position while the team pulls it along backward. 



POLE LINE CONSTRUCTION. 



785 



The following statement is made by the manufacturer of this 
wagon as to the economies incident to its use : 

"The majority of the telephone and construction companies are 
using eighteen men as a pole raising crew, and with a team of horses 
to pull poles into position, an average day's work is to set twenty to 
thirty-two poles; figuring the men at an average of $2 per day per 
man, gives $36 to set an average of twenty-six poles per day, or $1.83 
per pole for wages of the crew. 

"With the pole-raising derrick wagon some companies use eight 




FIG. 559.— DERRICK WAGON— POLE PARTLY RAISED. 



men in the crew and some prefer twelve, an average of ten men. 
An average day's work is to set forty-five to sixty-four poles. Fig- 
uring the average as $2 per day per man, gives $20 to set an average 
of fifty-five poles, or 36 1-3 cents per pole for wages of the crew, a 
saving of over $1.40 per pole. 

"These figures are subject to considerable variation on account oi 
varying conditions. On straight work where no wires interfere, as 
many as seventy-six poles have been set in a day with twelve men. 
and where wires are in the way and cause considerable trouble, the 
work is very much speedier than by the old method of pike poles. 

50 



786 



AMERICAN TELEPHONE PRACTICE. 



inasmuch as the pole is suspended in a manner that one or two men 
can have absolute control of the pole, moving it in any direction with 
comparatively little effort." 

If the soil is soft a foot-plate should be placed under the butt of 
the pole. This can be made by fastening together two 2 inch by 12 
inch pieces of oak or hard pine 2 or 2\ feet long, at right angles 
to each other. In case the soil is very soft, as in marshy districts, 
more elaborate means will have to be taken. The hole should be 
dug in such places much larger than in ordinarv instances, and a 




FIG. 560.-DERRICK \\ AGON— POLE RAISED AND BRACED. 



larger foot-plate may be inserted. A good plan, under these condi- 
tions, is to place in the bottom of the hole a layer, 6 inches deep, of 
concrete, and, after raising the pole, filling in the center hole to the 
surface of the ground with the same mixture, thoroughly tamped into 
place. For this purpose, and for other cases where concrete is needed 
in line construction work, the following formulas are given : 



Formula No. 1. 

Natural cement i part 

Sand 2 parts 

Broken stone 3 



Formula Xo. 2. 

Portland cement I part 

Sand 3 parts 

Broken stone S " 



POLE LINE CONSTRUCTION. 



787 



Broken stone is, as a rule, better than gravel, and stones of varying 
size, up to the size of an egg, are somewhat cheaper than stones of 
uniform size, because the small stones fill in the interstices between 
the large ones, and thus require less cement, while the concrete is 
just as strong. 

On a straight line three different kinds of strain must be pro- 
vided for, namely, the crushing strain, due to the weight of the 
wires; the side strain, due to wind pressure, and the strain in the 
direction of the wires. This latter is due to the tension in the wires 




: 



J 



-4- 



-* - :•■; 3-., ;;.■:■. * 



■■■" : "' : ''" •' ' ■,"?:'■'■■ •"- 



FIG. 561.-DERRICK WAGON-MOVING WAGON TO NEXT POLE. 



at the end of the line, or to wind pressure in the direction of the 
line, or to the tension in portions of the line caused by the falling of 
a pole or the breaking of a number of wires. In hilly country also 
considerable strain is caused in the direction of the line itself on a 
long down grade, due to the actual weight of the wires. The first 
two strains, that is, the crushing strain and the side strain due to 
wind, are at times very great, both being augumented by the forma- 
tion of a crust of ice on the wires and poles during sleet storms. 
Abbott cites cases where coatings of ice 6 inches in diameter have 



788 



AMERICAN TELEPHOXE PRACTICE. 



been formed on a Xo. 10 wire throughout its length. These, of 
course, are extreme cases, but coatings 2 inches in diameter are quite 
common. It is customary to provide for the crushing and side 
strains on a straight line by making the poles heavy enough to stand 
them without recourse to other methods, although on very heavy 
lines side guys are often used, even on straighway work. The sizes 
of poles given in Table VIII. is sufficient to insure against breakage 
in such cases under all ordinarv conditions. 

The strain in the direction of the wires should be provided for 
bv a method of bracing known as head-guying. This consists in 
running a guy wire from the base of one pole close to the ground 
to the top of the next, etc., for several poles in succession. About 
three poles should be guyed from the top of one to the butt of the 
next, and in the next three the order should be reversed, thus brac- 
ing the line in both directions. This, if repeated at intervals of I 
mile, will greatly strengthen the line against vibration in the longi- 




FIG. 562.— HEAD GUYING. 



tudinal directions caused by high winds or by the other causes men- 
tioned. On a down grade the head-guys should extend from the 
butt of the pole on the highest ground to the top of the pole below 
it. The method of head-guying is illustrated in Fig. 562. When 
the line is dead-ended at the termination of a lead, or for the pur- 
pose of connecting with an underground cable, the last three poles 
should be head-gaived bv running a gfuv wire from the bottom of the 
last pole to the top of the next, and so on for three poles. The 
■last pole should be guyed by planting a guy-stub at as great a dis- 
tance as possible beyond it. in the direct line of the poles, and firmly 
guying to it. It frequently happens in cities that sufficient room 
cannot be obtained for dead-ending a pole line in this manner, and 
under these conditions some sort of an anchor pole is necessary. 
Frequently room may be had by planting the anchor at a distance of 
perhaps 10 feet from the base of the pole. In this case the guy wire 
or rod should be made verv strong in order to successfullv stand the 



POLE LINE CONSTRUCTION. 



789 



excessive strain, and the anchor should be buried to a depth of per- 
haps 8 feet and weighted by a mass of rock and concrete. 

As an additional precaution a lattice-work of angle iron is in 
some cases used to reinforce the upper portion of the pole for the 
purpose of equalizing the pull on the guy rod without undue stress 
on any portion of the pole. In Fig. 563 is shown such a lattice-work, 
and also a method of anchoring a pole to be subjected to a severe 




FIG. 563.— DETAILS OF ANCHOR POLE. 



strain. Structural iron anchor poles are sometimes used for the ter- 
mination of very heavy leads, and these offer the neatest solution 
of the problem, but have the disadvantage of being extremely expen- 
sive. 

When a bend occurs in the line or when a branch lead is taken 
off at an angle, a side strain is exerted on the poles. These strains 
must be amply provided for by means of a system of braces which arc 
capable of exerting an opposite pressure to that of the pull of the 



790 AMERICAN TELEPHOXE PRACTICE. 

wires. This is usually done by means of guy wires, connected to 
the tops of the poles and extending in such direction as to bisect the 
angle of the bend which the line makes. On long curves a guy 
wire should be provided for each pole, and it is also well to head- 




FIG. 564.— SIDE GUY. 



guy each pole. Beginning at the center of the curve, head-guys 
should extend from the base of each pole to the top of the next pole 
in each direction from the center. The shorter the turn the greater 




FIG. 565.— Y-GUY. 



the strain, and the greater therefore must be the precaution taken 
to meet it. 

In Fig. 564 is shown an ordinary side guy used in light lines where 
there are but few cross-arms. 



POLE LINE CONSTRUCTION. 



•(?] 



Where more than four cross-arms are used a Y-guy (Fig. 565) 
should always be employed, as it takes the strain from both the 




FIG. 566.-GUY AT CROSSING. 



top and bottom arm. To guy from the top of the pole only, as is 
frequently done, causes the latter to bow toward the center of the 




FIG. 567.-PUSH POLE BRACE. 



curve at the lower cross-arm, and frequently causes the pole to break 
at that point, usually in the gain of the lower arm. On the other 



"92 



AMERICAX TELEPHONE PRACTICE. 



hand, to guy from the lower cross-arm usually causes a pole to bow 
in the other direction with the same result. 

Where possible, the anchor should be placed 20 or 25 feet from the 
base of the pole. When it is necessary to set the anchor 10 feet or 
less from the base of the pole, the pole should have a foundation 
formed of two planks 2 inches thick by 12 inches wide and 30 inches 
long set at right angles. 

To raise the guy wire to a considerable height in crossing a road a 
long guy-stub securely anchored may be used, as shown in Fig. 566. 




FIG. 568.— ANCHOR. 

Y\ "here it is not convenient to guy, a "push" pole brace, shown in 
Fig. 567, may be used. 

To properly anchor guy wires often requires a good deal of inge- 
nuity, and it is hard to lay down any definite rule, as they frequently 
have to be planned to meet the existing conditions. One of the 
most common methods, and a very satisfactory one, is shown in Fig. 
568. The anchor log should correspond in size to the depth of the 
excavation in accordance with the following table : 







Anchor Log. 




Depth of Excavation . 










Length. 


Diameter. 


4 feet 


4 feet 






6 inches 


4 " 


1 5 ■; 

17 " 
(5 " 






6 " 

8 " 
12 "■ 


3K" 


\l" 






9 " 








7 " 



POLE LINE CONSTRUCTION. 



793 



When an excessive strain is to be met, an anchor of several logs 
bolted together crosswise may be used. The anchor rod should 
be of wrought iron, from 6 to 10 feet long and from f to i^ inches 
in diameter, having an eye forged in one end and a heavy screw 
thread and nut on the other. The rod should pass directly through 
the anchor log and be secured by the nut, a heavy iron washer being 
placed between the log and the nut. All iron so used should be gal- 




FIG. 569.— GUY CLAMP. 



vanized. In extreme cases the log should be buried in a mass of 
concrete. 

A common way of anchoring a guy wire is to a guy-stub, which is 
usually formed of the stub end of a pole from 8 to 12 feet long, set 





FIG. 570.— GUY WIRE HOOK. 



FIG. 571.— HALLETT CLAMP. 



at an angle of approximately 90 degrees to the direction of the guy 
wire. 

The guy rope should be fastened to the pole by passing it twice 
around and clamping it by means of a malleable iron guy clamp. 
of which one is shown in Fig. 569. 

In order to protect the pole and to keep the guy wire from slipping 
down the pole, guy wire hooks are used such as are shown in Fig. 
570. 



794 AMERICAN TELEPHONE PRACTICE. 

Several good forms of guy wire clamps are on the market. These 
have from one to three clamping bolts according to the strain to 




FIG. 572.— "CINCINNATI" CLAMP. 



be borne. The single bolt clamp shown in Fig. 569 is known as the 
"Crosby" clamp. The Hallett clamp is shown in Fig. 571 ; the 




FIG. 573.-"A. T. & T." CLAMP. 



"Cincinnati" in Fig. 572, a three bolt "A. T. and T." clamp in Fig. 
573, and a three-bolt Cook clamp in Fig. 574. 

Tests of various single-bolt clamps show that one bolt or single 





FIG. 674.— COOK THREE-BOLT CLAMP. FIG. 675.— THIMBLE. 

"U"-bolt clamps sustain loads of from 600 to 2000 pounds without 
slipping of the guy wire; two-bolt clamps from 1000 to 4500 pounds, 
and three-bolt clamps from 1100 up to 6000 pounds. 



i M-^, 


a 

c 

1 


o 


;si 



§1 

r 

I 

r t 
|S 

z n 




795 



796 AMERICAN TELEPHONE PRACTICE. 

A thimble (Fig. 575) of galvanived steel is used for attaching the 
guy wire to the anchor rod, as shown in Fig. 569. 

The wire used in guying may consist of one or more strands of 
Xo. 9 or 10 B. & S. steel wire twisted together, but a better plan is 
to use the regular steel cables, thoroughly galvanized, furnished by 
the several reliable wire manufacturers. This has the advantage of 




FIG. 577.— HAND BARROW, 
r 

being more flexible, more easily handled, and, at the same time, 
stronger for its weight than the single strands of larger wire. The 
cable usually consists of seven Xo. 12 steel wires laid up with a 3-J- 
inch twist. 

In turning a sharp corner it is better to use two poles. Fig. 576 
shows several styles of double-pole corners, and also a method of 
taking off branch leads that is sometimes, but seldom, used. On 
corner work the poles should be heavier than the standard used, 




■- --rT-iiK- r Tiaj 



FIG. 578.— COME-ALONG. 

and should be guyed in a manner that will effectually brace them in 
all directions. The large cut in Fig. 576 shows the best method 
of guying, but the method shown in the lower left-hand corner may 
be used where there is room for only one guy stub and anchor. 

Sometimes instead of leading the bare wires around the bend, 
they are dead-ended on the corner poles in the same manner as at 



POLE LINE CONSTRUCTION. 797 

the end of a lead, and then connection made between the two leads 
by means of a lead-covered or rubber cable, suitable cable terminals 




ttQJ- 

FIG. 579.— IRON-WIRE TIE. 



(usually potheads) being used for connecting the cable wire with 
the bare wires. This construction is shown in the upper right-hand 
corner of Fig. 576. 

The same method has been used in taking off a branch lead from a 




FIG. 580.— HELVIN TIE. 



main lead. This is shown in the upper left-hand corner of Fig. 
576. The use of cable for corner and branch work frequently saves 
much complexity in difficult places, leaving the work much more 
open and clean. 




FIG. 681.— COPPER-WIRE TIE. 



After about a mile of pofes have been set and guyed, and the 
cross-arms, pins, and insulators put in place, the process of string- 



798 



AMERICAN TELEPHONE PRACTICE. 



ing, where but a few wires are to be run, consists in placing the 
reels on hand barrows, as is shown in Fig. 577, or on a cart, and 
paying them as they go, drawing the wire up to each pole separately. 

When, however, a large number of wires are to be run, the method 
is briefly as follows : 

Ten reels similar to that shown on the hand-barrow of Fig. 577, 



ssjnrct 




FIG. 582.— DEAD-ENDING IRON WIRE. 



are mounted on a wagon, so that ten wires may be pulled off at once. 
A running rope is pulled over the top cross-arms the full length of 
the section to be strung, and this rope runs through a snatch-block 
at the end of the section down to the base of the pole, where it passes 
through another snatch-block. A team of horses is then hitched 
to the end of the running rope and draws this back towards the 
wagon. The ten wires are fastened to what is known as a running 
board made in the form of a triangular frame, to one corner of which 
the running rope is attached. This enables the running board to 
slip over and by obstructions more easily. The side of the triangu- 
lar frame opposite the corner to which the running rope is attached, 




FIG. 583.-WESTERN UNION JOINT, FOR IRON WIRE. 

is provided with ten snap hooks to facilitate fastening the wires to be 
strung. Linemen place each wire in proper relation to the pin to 
which it is to be secured. 

A short hand line is usually fastened to one corner of the running 
board to keep it from turning over, and thus twisting the wires, and 
also to help it in passing trees and other obstructions. 



TOLE LINE CONSTRUCTION. 



'99 



When stringing wires on other than the top cross-arm the same 
method is followed, but as the running board passes each pole five 
of the wires are unsnapped and passed around the pole, and again 
attached to the running board after it has passed the pole. Another 
way, which prevents the unsnapping of the wires from the running 



FIG. 584.— McINTYRE SLEEVE. 

board, is to string on two cross-arms at once, five of the wires being 
utilized on one arm and five on the arm below it. After the wires 
are all in place each one is separately pulled up to the proper tension, 
and tied to the insulator at each pole. 

After the wires have been dead-ended at one end, experienced line- 
men are set to work pulling up the slack, attention being given to 
getting the same tension in all of the wires. This is quite an art, 
and it is important that it be done properly. The force is applied 
by attaching a wire clamp, commonly known as a "come-along," 




733 




yz sleeve 
FIG. 5S5.-DEAD-ENDING WITH McINTYRE SLEEVE. 

shown in Fig. 578, and pulling it up with a block-and-tackle or by 
hand. The tension depends on the kind and size of wire, on the 
distance between the poles, and on the temperature at time of the 
stringing. The amount of tension put on each wire is usually taken 
as about one-third the breaking strength of the wire, which may be 
found from the wire tables. The other method is to allow a cer- 



800 



AMERICAN TELEPHONE PRACTICE. 



tain sag or distance between the center of the span and the straight 
line between the points of support. 

Table XIII., which is taken from Roebling's handbook on 
"Wire in Electrical Construction," gives the sag in inches for the 
various lengths of span at different temperatures, these figures being 
based on the use of good hard-drawn copper wire. 

TABLE XIII — Amount of Sag in Spans. 









Spans in Feet. 






Temperature 

in 

Degrees 

Fahrenheit. 


75 


IOO 


115 I30 


150 


2GO 








Sag in Inches. 







—30 


I 


2 


2/2 


3H 


4^ 


8 


— IO 


I* 


2^ 


3 


3U 


5 


9 


lO 


I# 


2/s 


3 l A 


4 3 A 


5H 


IO# 


30 


I 3 / 


3 


4 


sV& 


bU 


12 


60 


2^ 


4X 


5'A 


7 


9 


i5# 


80 


2>Vs 


5^8 


7 


8/s 


11X 


18X 


IOO 


aVz 


7 


9 


11 


14 


22X 



The tying of wires to the insulators is an important matter, and 
there are several different methods of doing it. The ordinary 
method used almost since the beginning of line construction is 
shown in Fig. 579. This is now used in the tying of iron wires only. 
The line wire merely passes along the side of the insulator and 
should not be bent, being held in the groove by a tie wire, twisted 
around it in opposite directions at each end, as shown. The tie wires 
are, as a rule, about sixteen inches long, and made of slightly 
smaller diameter than the line wire itself, especially in case of very 
heavy wire. 

Another method, known as the Helvin tie, is shown in Fig. 580. 
This has been used with considerable success with hard drawn cop- 
per wire, but is not now used so far as known. In this the tie wire 
is first wrapped around the insulator and twisted once or twice on 
itself, after which the ends are twisted around the line wire as before. 

Still another method of tying the wire to the insulator is shown in 
Fig. 581, this being now used exclusively in copper wire work. In 
this, as in the first method, the line wire is laid in the groove of the 
insulator, and the tie wire is passed entirely around the groove, one 
end passing down over the line and the other end up under it, the 
twist being made as shown. 

Where an iron wire is dead-ended it is simply passed once around 



POLE LINE CONSTRUCTION. 



801 



the insulator and twisted five times upon itself, the twist beginning 
at a distance of about two inches from the insulator. The method of 
making this tie is shown in Fig. 582. In the case where trans- 







FlGi 586.— LILLIE JOINT. 

positions are to be made the free end of the wire should be left long 
enough to pass over and make connection with the other side of the 
circuit. 

The joining of wires is a matter which has received much at- 



^CMMM^ 




,•; 




FIG. 5S7.-LILLIE JOINT. 

tention. The old style of joint, and one which gives satisfaction 
for iron wire, is shown in Fig. 583. This is known as the Western 
Union joint, and is made by placing the two ends side by side and 
clamping them with a hand vise or with a heavy pair of pliers. 



802 AMERICAN TELEPHONE PRACTICE. 

With another pair of pliers, the free end of each wire is twisted 
tightly around the other wire, as shown. This is used exclusively 
for joining iron wires. 

Another method of joining wires, known as the Mclntyre sleeve 
joint, is shown in Fig. 584, this being now almost universally used 
in joining copper wire. The sleeve for making this joint consists 




FIG. 588.— TRANSPOSITION. 

of two copper tubes soldered together and having a bore corres- 
ponding to the sizes of the wire to be joined. The ends of the wire 
are passed in opposite directions through these tubes and are then 
grasped at each end with a special tool for the purpose, and given 
three distinct twists. This joint is now widely used in practice and 
is very convenient because the use of solder is not required in order 
to make it perfect. 

In Fig. 585 is shown the method of dead-ending a copper wire 
with a half length of Mclntyre sleeve. 

Still another connector, not so well known, is the Lillie joint, 
this being shown in Fig. 586. The connector in this consists of a 



dB 



FIG. 589.— POLE-TRANSPOSITION. 

sheet of copper curved longitudinally in opposite directions. The 
wires are slipped in each curve of the strip and twisted in opposite 
directions, as in the Mclntyre joint. This joint has not come into 
such extensive use as the Mclntyre sleeve, but should prove efficient. 
Fig. 587 shows how this sleeve may be applied in taking off branch 
wires, as in the case of attaching bridging telephones to a line. This 




POLE LINE CONSTRUCTION. 



803 



joint is advantageous in this respect, in that it is not necessary to 
cut the line wire to apply it. 

The method of making transpositions on copper wire is shown 
in Fig. 588. This is done by cutting the wires on the pole side 
about 20 inches from the cross-arm, and slipping on each a half 
Mclntyre sleeve, with which the wires on the cross-arm are dead- 
ended, one in the lower groove and one in the upper groove, of the 
respective transposition insulators, leaving the ends projecting. 




FIG. 590.— TELEPHONE AND POWER CIRCUIT. 



About six feet of slack is then joined to the wires on the cross-arm 
side by using whole Mclntyre sleeves. Half sleeves are then slipped 
on and the wires dead-ended in the vacant grooves of the transposi- 
tion insulators. In dead-ending the stationary clamping tool should 
be held next to the insulator, so that the twists will be made in the 
long section. Fig. 389 shows the method of crossing over the wires 
where the two wires transposed are on the opposite sides of the 
pole. In making transpositions a good, though more expensive 
way, is to use double cross-arms at the transposition poles, dead- 



804 AMERICAN TELEPHONE PRACTICE. 

ending the wires on each and bridging across by bridle wires in 
much the same manner as shown. 

It is frequently necessary to run a telephone line on the same 
poles with a high-tension power circuit. Induction from the power 
wires is of course under these conditions very likely to render con- 
versation impossible, especially if the current in the power circuit 
is alternating. Fig. 590 shows the details of a pole thus equipped, 
the two insulators on brackets being for the telephone line. The 
latter should be of No. 12 B. & S. copper, and transposed every 
three poles. In this way a fairly quiet line may be obtained under 
the most unfavorable circumstances. 



CHAPTER XL. 
AERIAL CABLE CONSTRUCTION. 

The tendency of telephone practice in cities is to bunch in the 
cables, the line wires following the same route, so that many wires 
may be placed in a very restricted space. It may be added that 
there is also a strong tendency toward the placing of these cables 
underground, this being due in great measure to the protests of the 
public against all overhead electrical construction, and to the de- 
creased cost of maintenance of underground work. Overhead ca- 
bles are however used to a large extent, and there probably always 
will be conditions under which it will be advantageous to use them. 

The overhead or aerial cable presents many advantages over the 
use of. bare wires. In many districts it would be absolutely impos- 
sible to handle the required number of wires without the use of ca- 
bles on account of lack of space for one hundred or more wires, 
which alone would require the use of a pole line carrying at least 
twenty cross arms, may be crowded into a cylindrical space less than 
two inches in diameter. Besides this the cost of construction is in 
many cases greatly cheapened; the, lines are rendered far more 
sightly; the danger of crosses from high tension or other wires is 
greatly reduced; the liability to injury in winds and storms is les- 
sened; and much less obstruction is offered to firemen in the per- 
formance of their duties. 

The use of cable brought new electrical problems, for obviously 
the placing of so many conductors in such a contracted space tended 
greatly to change the electrical properties of the circuits. The ad- 
vent of cable rendered the use of smaller wires possible, and this 
necessarily brought about greater resistance per lineal foot. The 
bringing of the two sides of the line close together and close to manv 
other line 'circuits tended to increase the electrostatic capacitv of 
each circuit, thus bringing about a greater shunting effect on the 
voice currents. Again, the bringing of the wires so close together 
brought in problems relating to the insulating of the various wires 
from each other, making necessary a continuous solid insulator of 
such a nature as to be capable of holding the wires apart at all por- 
tions of their length. Obviously, also, the close spacing of wires 

S03 



806 



AMERICAN TELEPHONE PRACTICE. 



rendered necessary greater precautions for the prevention of induc- 
tion between them which would give rise to cross talk and similar 
trouble. 

Briefly stated, the electrical problem in cable construction has 
been the production of a cable in which the conductors will be as low 
in resistance as possible, in which the electrostatic capacity between 
the various conductors will be as low as possible, in which the re- 
sistance between the various conductors shall at all times be as high 
as possible and in which the relations between the various conduc- 
tors are such that inductive action between the various circuits will 
be practically nil. 

Besides the electrical requirements there are certain obvious 
mechanical ones which will be dealt with more specifically later on. 

In the earlier days of telephony rubber was considered the best 
insulating material for the wires in cables. Cables so constructed 
are now used only in special and comparatively rare cases. Rubber- 
covered cables give excellent results as to insulation and durability; 
but a serious objection to their use for telephone work is that their 
electrostatic capacity is very high. This is due to the fact that while 
rubber is a splendid insulator, its specific inductive capacity is much 
higher than that of some other insulators. 

Table XIV. shows the diameter and weight of rubber-covered 
cables of various numbers of pairs, the conductors being in each 
case of No. 18 B. & S. G. wire, tinned, and double coated with 
rubber. 

TABLE XIV. — Aerial Cable. Rubber-Covered Wires. 



Number of Pairs. 


Number 
of Conductors. 


Diameter in Inches. 


Weight per 1000 Feet 
in Pounds. 


3 


6 


9 


175 


5 


IO 


11 


256 


IO 


20 


TS 


452 


15 


30 


I 


633 


20 


40 


1% 


813 


25 


5o 


I* 


994 



Dry air is the most desirable insulator with respect to electrostatic 
capacity and of course air is the insulator mainly used in bare wire 
work. The reason for this superiority is that the specific inductive 
capacity of air is lower than that of any other substance that is at all 
practical to use. At first it was not thought possible to rely upon 
air at all in cable construction, but a great improvement over rubber 



AERIAL CABLE CONSTRUCTION. 



807 



cable was brought about by the use of paper insulation between the 
individual conductors. In the earlier forms of cables so con- 
structed the wires were wrapped with paper which was afterwards 
impregnated with some insulating material, such as paraffin, hav- 
ing relatively low specific capacity. Such cables were once largely 
used, being known as saturated paper cables. 

It was found that by aerating the paraffin thus used in saturated 
cables with dry carbolic acid gas, the electrostatic capacity between 
the conductors was reduced as much as 15 per cent. 

In order still further to reduce the capacity, what are known as 
dry core cables have been introduced and have been adopted almost 
universally in telephone work. These are usually formed in this 




FIG. 591.— DRY-CORE PAPER CABLE. 



country by wrapping the separate conductors with two layers of 
dry paper loosely laid on. Some times only a single wrapping is 
used. Two wires thus insulated are formed into a twisted pair, 
the length of a complete twist being about three inches. These 
pairs are formed into a core in layers, the twist in the successive 
layers being reversed in direction. The paper is very porous, which 
fact, together with the loose construction, allows the permeation of 
dry air throughout the structure. 

After the core is formed it is served with heavy manilla paper and 
the whole is enclosed in a lead sheath, usually about one-eighth oi 
an inch in thickness. A piece of such cable from which the lead 
sheath has been partially removed is shown in Figure 501. 



808 AMERICAN TELEPHONE PRACTICE. 

The method of applying the lead sheath is an interesting feature 
of cable manufacture. In the earliest lead covered cables, the core 
was formed complete in as long a length as could be shipped on one 
reel when completed, then sections of lead pipe of proper diameter 
and about fifty feet long were slipped over the core and soldered to- 
gether at their junctions. Later, the present method of applying the 
lead covering was adopted. This is to pass the core direct from its 
drying oven into a lead-press of the type used in making lead pipe, 
from which press, charged with just molten lead, the core and its 
covering pipe emerge as a finished thing. The exact way in which 
the lead gets formed around the core can be understood best by 
conceiving the core to pass through the bottom of a tank filled with 
just-flowing lead, and then passing out through the side of the tank 
through a ring-shaped die. 

Another way of forming a twisted pair in cable work is to lay 
the two wires upon opposite sides of a strip of paper and then 
twisting the two together with the paper strip between them. The 
pair is afterwards served with a single wrapping of paper, forming 
a complete tube around it. This method is not widely used in this 
country, the separate wrapping or wrappings on each individual 
wire having come into almost universal use. 

The saturated cable had the advantage of not being so susceptible 
to moisture as the dry core cable, but its electrostatic capacity was 
much higher, and principally on that account it has been abandoned. 
The capacity between a single wire and all of the other wires 
bunched together and connected to the sheath is for a certain type 
of dry core cable about .080 microfarads per mile. For a saturated 
cable of the same dimensions the electrostatic capacity would be 
from .15 to .20 microfarads per mile, or even higher. This illus- 
trates well the difference in electrostatic capacity between a saturated 
and a dry core cable. 

The sizes of conductors ordinarily used in telephone cables vary 
from No. 19 B. & S. G. down to No. 22 B. & S. G., Nos. 19, 20, and 
22 being standard sizes. There has been recently a tendency in 
large work to use No. 24, but this has not as yet become well stand- 
ardized in practice. Nos. 20 or 22 are most frequently used in 
aerial construction. 

For toll line work where the lowest possible electrostatic capacity 
and resistance is required, and yet where the conditions are such that 
the wires must necessarily be bunched into cables, No. 16 B. &. S. G., 
or even as large as No. 13 B. W. G v are sometimes used in paper 



AERIAL CABLE CONSTRUCTION. 809 

cables. In these cases, in order to further reduce the capacity, the 
wires in the cable are very loosely laid together, thus forming a 
very soft and yielding bunch over which the lead sheath is placed. 

The American Telephone and Telegraph Company's specifica- 
tions allow as a maximum conductor resistance per mile of cable for 
various sizes of conductor the following: 

No. 22, B. & S. G 95 ohms per mile. 

" 20, " 60 

" 19, " .' 47 

" 16, " 2 3 y 2 

" i 3 ,B.W.G 6 T 3 o 

These figures are for a mile of single conductor, and they take 
into consideration the allowance for the increased length of con- 
ductor due to twisting the wires in pairs and to the twist of the pairs 
in the cable. 

The use of two wrappings of paper on a conductor rather than 
one, and the loose bunching of wires in a cable rather than squeez- 
ing them tightly together, tends to produce higher insulation be- 
tween the conductors and at the same time to bring about a lower 
electrostatic capacity. In the best cables two wrappings of paper 
are used and the wires are as loosely bunched as is compatible with 
the space they are to occupy and the size of the investment that it 
is desired to make. It is obvious that a considerable amount of 
money may be saved by squeezing the wires tightly together, thus 
making them go in smaller space and requiring less sheath to 
cover them. The sizes of the cable vary widely although the same 
amount of insulating material, the same size of conductor and the 
same thickness of lead sheath is used. With these factors fixed the 
size of the cable will be governed largely by the requirements as to 
electrostatic capacity and insulation. 

The usual method of measuring insulation resistance is to measure 
the insulation between one wire and all of the other wires in the 
cable bunched together and connected to the sheath. The usual 
requirements specified for insulation resistance is that it shall be 
not less than 500 megohms per mile measured as stated above. 

It is evident that this degree of insulation in a dry core cable can 
only be maintained by keeping the insulating material within the 
cable perfectly dry, and this is of course dependent upon the lead 
sheath being kept intact. This point will be considered later on in 
this chapter. 

As already stated, the size of the cable may be largely governed 
by the requirements as to electrostatic capacity, other things being 



810 AMERICAN TELEPHONE PRACTICE. 

equal. The method of measuring capacity, therefore, becomes im- 
portant. The old and until now standard method of measuring elec- 
trostatic capacity was, as has already been stated, to measure the 
capacity of each wire against all of the other wires bunched and con- 
nected with the sheath. A new method has recently come into 
vogue, which is a test for mutual electrostatic capacity, that is, 
the capacity between the two wires of a pair, these two wires being 
disconnected from all others, but all of the other wires being con- 
nected together and connected with the sheath. It is quite evident 
that the mutual capacity between a pair of wires will be less, under 
ordinary circumstances, than the capacity between any one wire 
and all of the others. The equivalent capacities according to these 
two methods of measuring are as follows, the capacity of the same 
cable according to each of the methods being given for several types 
of cable: 

Capacity one Wire to all Others. Equivalent Mutual Capacity. 

.08 .054 

.10 .066 

.12 .08 

.14 .O93 

.16 .107 

.18 .12 

Specifications usually state that the cable sheath shall be com- 
posed of an alloy of lead and tin, the amount of the latter being 
not less than 3 per cent, of the entire mixture. This requirement 
has been made because it has been found that such an alloy is not 
so susceptible to chemical action as lead alone, which is an important 
consideration in underground work. The Standard Underground 
Cable Company appears to advocate the use of pure lead sheath for 
all cases, the sheath being treated, after forming, with an external 
coating of pure tin. They argue for this that the tin when mixed 
with the lead makes the sheath brittle, and that the tin will be most 
advantageous if all of it is placed on the outside. This argument 
is given some further weight in view of the fact that it has been found 
difficult to secure an absolutely uniform alloy of tin and lead, result- 
ing in some parts of the sheath having an excess of tin and being 
then unduly brittle. 

The general practice at the present time is to use an alloy of 3 
per cent, tin for underground work and pure lead for overhead work. 

The use of a braiding saturated with a mixture, previously com- 
pounded, placed over the outside of the sheath was once advocated, 
and much of such cable was formerly used. It has been found, 



AERIAL CABLE CONSTRUCTION 811 

however, that the disadvantage of the outside braiding outweighed 
its advantages in either overhead or underground work. The locat- 
ing of punctures in the sheath of aerial cables is made much more 
difficult by the use of this braid, for when the sheath is bare they 
may be located by mere external inspection. Very often the braid- 
ing considerably increases the cost of the cable, and its only advan- 
tage has been in tending to prevent abrasion, which need not occur 
if the cable is properly supported. In underground work the braid- 
ing effected but a poor protection for the sheath during the drawing- 
in process and practically none against chemical action. After the 
braiding rots, as it does, the pieces hanging from the cable present 
an unsightly appearance in the case of overhead work, and may 
serve to thoroughly clog up the conduit in underground work in 
attempting the subsequent withdrawal of the cable. 

A new objection to the old external braiding for cables has re- 
cently developed. Mr. John Hesketh, electrical engineer for the 
State of Queensland, Australia, in a very valuable contribution to 
the proceedings of the International Electrical Congress at St. 
Louis, in September, 1904, entitled "A New Danger to Lead- 
Covered Aerial Cables," has shown that many of the formerly 
unaccountable punctures in lead cable sheaths is due to the work 
of certain insects, which, either for the purpose of finding a 
secure place in which to deposit their eggs, or in the hope of find- 
ing food, bore through the sheaths as they would through the bark 
of a tree. One form of these is developed from eggs laid on the 
cable sheath under the braid, and these insects as soon as hatched 
start at once to bore into the sheath. The danger to cables is thus 
much heightened by the presence of the braid, as it is found that the 
eggs are not so often deposited upon the bare sheaths. This ac- 
counts in a large measure for the mysterious presence of holes in 
cable sheaths immediately adjacent to or under the cable hangers, 
and as is well known, these holes have usually been attributed to 
some mysterious electrostatic action, or to an electrical discharge 
from the conductors to the hangers. The facts in Mr. Hesketh's 
paper merit the serious attention of all telephone men, as Mr. Hes- 
keth states that the insects capable of this damage are many in kind 
and are by no means confined to Australia. 

Table XV. gives the outside diameter and weight per thousand 
feet of the various sizes of lead covered paper cable each of 100 pairs 
manufactured by a prominent firm. The conductors are No. 10 
B. & S. G., each served with two layers of paper. The average elee- 



812 



AMERICAN TELEPHONE PRACTICE. 



trostatic capacity is .080 microfarads, each wire being measured 
against all the others and the sheath. The diameters and weights 
of such cable will differ largely with various methods of manufac- 
ture, and with different capacities, or other electrical properties. 



TABLE XV.— Aerial Cable. 



Number of 


Outside Diam- 


Weights per 1000 


Number of 


Outside Diam- 


Weights per 1000 


Pairs. 


eter, inches. 


Feet in Pounds, 


Pairs. 


eter, inches. 


Feet in Pounds. 


I 


ft 


214 


30 


IjV 


2.748 


2 


302 


35 


*X 


2,985 


3 


% 


515 


40 


iA 


3,176 


4 


% 


629 


45 


i# 


3,365 


5 


747 


50 


itf 


3.678 


6 


w 


877 


55 


i« 


3,867 


7 


\\ 


912 


60 


i# 


4,055 


10 


H 


1,214 


65 


T 1 5 

J T3" 


4,241 


12 


it 


1,375 


70 


2 


4,430 


15 


1 


1,566 


80 


2^ 


4,804 


18 


1* 


i,758 


90 


2X 


5,l8o 


20 


i# 


1,940 


100 


2^8 


5 505 


25 


iA 


2,232 









Before passing to the methods of supporting aerial cable, it is 
well to state briefly some facts concerning the talking qualities of 
conductors in cables. Preece's famous K R law states in substance 
that when the product, K R, formed by multiplying the elec- 
trostatic capacity, K, of a circuit expressed in microfarads by the 
resistance, R, expressed in ohms, exceed 15,000, speech becomes 
impossible. And that for smaller values of this constant the talking 
quality of a circuit is in direct proportion to this product. This 
law has apparently been disproven so far as limiting value of K R 
is concerned, as there are many circuits in the United States in com- 
mercial use whose K R value is much over 15.000. For instance, 
the New York-Chicago circuits have a K R value of something like 
25,000. Mr. Preece has, however, pointed out that the accuracy of 
the law when approaching the limiting values of speech transmission 
has not been disproved, and that in the case of the very long circuits 
which seemed to disprove the law, there was some question as to 
whether the electrostatic capacity was determined with accuracy. 
It may be said, however, on the other hand, that when the electro- 
static capacity of 1000 mile line is measured a value is arrived at 
not greatly different from that secured by measuring one mile of 
the same kind of line and multiplying it by 1000. It is not, there- 



AERIAL CABLE CONSTRUCTION. 813 

fore, altogether clear why the measurements of the value of K on 
these circuits was not a true determination. It is very fair to say 
that with ordinary methods of measuring the value of K, the K R 
law does not hold as to the limiting values of speech transmission. 

The K R law is very fairly accurate for other than limiting 
values, and it may be said that the talking value of a circuit is in 
inverse proportion to the K R value. With this in view it is easy, 
with a given standard of speech transmission in mind, to determine 
roughly the spcifications for a cable of given length which is desired 
to give this standard transmission. If it is desired to find out what 
length of No. 20 B. & S. G. cable with an electrostatic capacity of 
.10 m.f. per mile, will give the same transmission as 10 miles of No. 
19 cable, having an electrostatic capacity of .08 m.f. per mile, it is 
only necessary to figure the K R value of the cable taken as a 
standard and to take such a length of the new cable as will produce 
the same K R value. 

Consideration of this method will often throw much light on the 
size and kind of cable that should be used in specific cases. 

There are other considerations than those of mere talking 
efficiency which may enter into the determination of the proper 
cable to use in any given place, and it is not improbable, in view of 
changes through which telephone apparatus proper is now going, 
that other considerations may prove to be of more weight. These 
other considerations are those relating to the operation over the 
cable circuit of exchange apparatus, such as relays. Particularly, 
the current supply necessarily involved in operating trunk apparatus 
may have some influence on the size of conductors. Again, the ad- 
vent of the automatic telephone switchboard, using as it does rapidly 
recurring impulses of current for the operation of the switches, 
makes it more probable that not only the ohmic resistance of the 
line, but its electrostatic capacity as well, may prove an important 
factor in the proper operation of the switches themselves. 

Aerial cables are supported on steel rope stretched tightly be- 
tween poles or other supports. This is stranded wire, and has the 
trade name of messenger wire. The use of messenger wire is 
necessary because the cable has not the requisite strength to support 
its own weight. For the heavier cables the messenger wire is 
usually composed of seven No. 8 steel wires twisted together into a 
rope. Table XVI. gives the common sizes of two different grades 
of messenger wire together with their diameters, weights and break- 
ing strengths. 



814 



AMERICAN TELEPHONE PRACTICE. 
TABLE XVI. 







EsTiiiATED Breaking Strength, Pounds. 


Diameter in 32ds 


Weights per 100 






of an inch. 


Feet, Pounds. 










Ordinary. 


Special. 


16 


51 


8,320 


16,640 


15 


48 


7,500 


15.000 


14 


37 


6,000 


I2.000 


12 


30 


4,700 


9,4O0 


10 


21 


3-300 


6,600 


9 


18 


2,6oo 


5,200 


8 


ny 2 


1,750 


3,500 


7 


*U 


1,300 


2,600 


6 


W 


1,000 


2,000 


5 


A% 


750 


1,400 


4 


2X 


375 


750 


3 


2 


320 


640 



For first-class work the wires of special strength, as given in the 
right-hand column, should be employed. 

Table XVII. is useful in determining the size of cable that any 
messenger wire may safely carry, for any given length of span. 
By referring to the table giving the weights per thousand feet of 
cable and knowing the maximum length of span the size of mes- 
senger wire may be determined. It is not customary or necessary, 
however, to compute the size of messenger wire separately for each 
cable. The one-half inch messenger is largely used for all but the 
lightest cables. A cable man may always ride this with safety, 
which is not the case with some of the smaller messenger wires. 

TABLE XVII.— Ordinary. 






•|- s 



Spans in Feet. 



no 120 


125 


130 


140 


150 1 175 



200 



Weights of 1000 Feet of Cable. Pounds. 



16 
15 
14 
12 
10 

9 

8 

7 
6 



2,818 


2,516 


2,263 


2,152 


2,050 


1,867 


1,709 


i,39i 


2,520 


2,247 


2,020 


1,920 


1,827 


1,663 


1,520 


1,234 


2,030 


1,812 


1,630 


i,55o 


1,476 


i,344 


1,230 


1,001 


1,580 


1,409 


1,266 


1,204 


1,146 


1,043 


953 


774 


I,IIO 


899 


890 


846 


805 


733 


670 


544 


860 


765 


680 


652 


620 


563 


513 


414 


585 


765 


468 


445 


423 


3^5 


352 


285 


433 


385 


346 


329 


313 


284 


260 


2IO 


337 


300 


270 


257 


245 


223 


2 4 


165 



1,154 
1,130 

900 
640 
450 
340 
235 
172 

137 



AERIAL CABLE CONSTRUCTION. 
Special. 



815 



-»T3 

.2 c 



16 
15 
14 
12 
10 

9 

8 

7 
6 



Spans in Feet. 



no 


120 125 


130 


140 


150 



174 



200 



Weights of 1000 Feet of Cable. Pounds. 



6,146 


5,432 


5,036 


4,814 


4,5io 


4,244 


3,928 


3,292 


5,520 


4974 


4,520 


4,320 


4,i34 


3,808 


3,52o 


2,948 


4,430 


3,994 


3,630 


3,47o 


3,322 


3,058 


2,830 


2,372 


3,460 


3,"8 


2,832 


2,708 


2,592 


2,386 


2,206 


1,848 


2,430 


2,008 


1,990 


1,902 


1,820 


1,820 


i,550 


1,298 


1,900 


1,710 


i,54o 


1,484 


1,420 


1,306 


1,206 


1,008 


1,285 


i,i57 


1,051 


1,005 


961 


885 


819 


685 


953 


857 


778 


745 


712 


655 


607 


507 


737 


663 


603 


577 


553 


509 


472 


393 

1 



2,818 
2,520 
2,030 
1,580 

I,IIO 

860 
585 

473 
337 



The messenger wire may be supported in various ways, one of 
which is to bolt what is called a messenger wire clamp to the side of 
the pole, which in turn serves to clamp the messenger wire in place. 

592. A form of messenger clamp, 



This method is shown in Fig 




FIG. 592.— MESSENGER WIRE CLAMPS. 

known as the Stroud clamp, shown in two views at the right of 
Fig. 592, has the advantage of requiring but one bolt to fasten the 
clamp block in place, a projection on the clamp block which passes 
into the main portion of the support serving in lieu of the second 
bolt. Another advantage of this type of clamp is that the Uo\\ 



Slo AMERICAN TELEPHONE PRACTICE. 

never need be taken entirely out, which prevents the separation and 
possible loss of the clamping block and bolt. 

Where more than one or two cables are to be supported from a 
pole, cable cross arms are often used, these being of angle iron con- 
struction, bolted to the pole as an ordinary cross arm and braced by 
cross-arm braces in some such manner as is shown in Fig. 593. 
If it is desired also to carry one or more bare wires, wooden cross 
arms may be secured within the angle iron arm as shown in this 
Figure, on which wooden arm ordinary pins and insulators may be 
secured. 

The cable is suspended from the messenger wire in several dif- 
ferent ways. Sometimes it is suspended by binding it to the mes- 
senger wire by strong tarred marlin. The marlin is wrapped around 
both cable and messenger wire, usually in two directions to give it 




FIG. 593.— CROSS-ARM FOR MESSENGER SUPPORT. 

greater security. This method is still largely used by the Western 
Union Telegraph Company in supporting its cables. 

The method now most extensively used by telephone companies 
in supporting aerial cable is by means of cable hangers adapted to 
tightly encircle the cable sheath and provided with a hook to slip 
over the messenger wire. One of the best forms of cable hanger 
is that shown in Figure 594. This consists merely of a loop of 
marlin, to one end of which is attached a galvanized steel wire 
hook. The method of attaching this to the cable is clearly shown in 
the figure. The marlin hanger has been much criticized on the 
ground that it would subsequently rot and allow the cable to fall, 
and many forms of all metal hangers have been devised to take its 
place. Some of these involved the use of a zinc band which was 
placed around the cable and supported by the wire hook. They 



AERIAL CABLE CONSTRUCTION. 



817 



were very satisfactory, but the marlin is perhaps the standard of to- 
day, it having demonstrated its capability of withstanding the effects 
of the weather for many years. 

It is well before hanging the cable to make tests for continuity of 




MARLIN HANGER. 



the conductors and for insulation of its resistance and capacity be- 
fore the cable is unreeled, so that any difficulty that may exist may 
be attributed to the fault of the manufacturer or to the transporta- 
tion company. For facilitating such tests the cable is usually placed 
upon reels in such a manner that both of its ends are available for 
such tests. 

When the cable arrives both of its ends are sealed to prevent the 




FIG. 595.-RUNNING UP CABLE. 



entrance of moisture, and alter testing, the end, should be carefully 
re-sealed in a manner that will be described later 

The method of hanging cables is shown in Pig 595 The en I 
of the supporting strand after passing over the last messenger wire 
clamp D is firmly secured to a guy-stub set in the ground at (' 



818 



AMERICAN TELEPHONE PRACTICE. 



The reel on which the cable is coiled is placed in line with the mes- 
senger wire and a few feet beyond the stake as shown. One or 
more grooved pulleys, C C, mounted on suitable supports are placed 
between the reel and stake in such a manner as to support the cable 
when it is paid out. A stout rope, or better, a small wire cable, is 
previously hung on pulleys or hooks below the cross arms of the 
entire stretch over which the cable is to be drawn. One end of this 
it attached to the end of the cable, while the distant end is attached 
to a capstan or other form of windlass. As the cable passes over 
the rollers, C C, the hangers are attached and are placed one by one 
upon the inclined messenger wire as they reach the point, B. As 
the cable progresses, linemen stationed on each pole lift the hangers 




FIG. 596.-CABLE REEL JACKS. 

over the messenger wire clamp or cross arm as they pass. In this 
way the entire length of cable is drawn up to and along the stretch 
without subjecting any portion of it to an undue strain. The 
hangers are usually attached at distances of from twenty-four to 
thirty inches, according to the size of the cable. The work is some- 
what expedited if, during the drawing up of the cable, only about 
every fifth hanger is hooked over the messenger wire. This reduces 
the labor of the linemen in lifting the hangers over the support. 
When, however, the forward end of the cable reaches the beginning 
of the last span, the signal should be given to all linemen stationed 
on the poles to hook on all the hangers as they pass, and in this way 
all of the hangers will be secured in place throughout the entire 
stretch without going out over the line afterwards, 



AERIAL CABLE CONSTRUCTION. 



819 



In Fig. 596 is shown a cable reel support, provided with screw 
actuated lifting jacks so that the reel may be readily lifted from the 
ground to allow its rotation. 

The method of hanging cable shown in Fig. 595 is subjected to 




FIG. 597.-CABLE ROLLER. 

a disadvantage when all-metal cable hangers are used, in that the 
sliding of the hanger hooks along the messenger wire tends to loosen 
the hangers on the cable, sometimes to such an extent that several 
of them become bunched at one point of the cable. 

A modification of this method which does away with the necessity 
for a man on each pole to lift the hangers over the messenger wire 





FIG. 598.-CABLE TROLLEY. 

support involves the use of the apparatus shown in Fig. 596. A 
bracket, such as shown in this figure, carrying a grooved pulley and 
a temporary support for the messenger wire, is temporarily attached 
to each pole. As the cable passes each pole the pullev serves to 
raise the cable so that the hanger will readily pass over the tem- 
porary support. When the cable is in place the brackets are re- 



$20 



AMERICAN TELEPHONE PRACTICE. 



moved and the messenger wire secured to the pole in any ordinary 
manner. 

Still another way of drawing up aerial cable is by means of so- 
called "cable trolleys." These consist of grooved pulleys such as 
are shown in Fig. 598, which may be secured to the messenger 




~rfJTftnt^fraiam*iai» : i.i^ 1 ■* 



FIG. 599.— CABLE SPLICING. 

wire at frequent intervals, and over the pulleys of which cable is 
drawn, the hangers not being fastened to the messenger wire at 
all. After the cable is in place the ordinary hangers may be ap- 
plied and the cable trolley removed. 

When it becomes necessary to splice a cable, the greatest care 
should be taken that no moisture be allowed to enter while the splice 
is being made, and that the splice shall be so thoroughly sealed at 
the end of the operation that there will be no possibility of the sub- 
sequent entrance of moisture. A suitable staging should be erected 
on the pole where the splice is to be made if it is possible to bring 
the splice within reach of the pole. This can always be provided for 
in new cable, but sometimes in repairing a leak it is necessary to 




Wire Joints 



Completed wire joint 



FIG. 600.-DETAILS OF WIRE SPLICE. 



make these splices from a car suspended from the messenger wire. 
When all is ready the lead sheath of each end of the cable to be 
spliced should be cut away for a distance of about two feet, the ends 
of the cable having previously been cut off square. The cable ends 
then appear as in Fig. 599. The wires should then be bound 



AERIAL CABLE CONSTRUCTION. 



821 



tightly together where they emerge from the lead sheath, using cot- 
ton twine or wicking. This winding should be extended close up 
to the end of the sheath. This is to prevent paraffin applied in 
the next operation from following along the core of the cable within 
the sheath. 

The next operation is to dip the end of the cable so prepared in 
hot paraffin, which must be heated above 212 degrees F. The 
cable ends should be held thus immersed until all bubbling ceases, 
when they should be taken off. This preliminary "boiling out" is 
to prevent moisture from getting into the cable while splicing, and 
should extend well over the cotton wrapping already applied. 




FIG. 601.— CABLE SPLICING. 



A sleeve, consisting of a lead tube of the following dimensions for 
various sizes of cable, should then be slipped over one of the cable 
ends and back out of the way. 

Lead Sleeves — Straight Joints. 

150 pair cable 30 inches x 3 inches x yi inch 



120 

ICO 

90 
60 
50 
30 
25 



28 
28 
28 
28 
28 
28 

2S 



" X23/ 


■ *X '■ 


11 X2^ 


' x* " 


" X2/ 2 


■ v 8 " 


" X2X 


' *A •< 


" X2 


' *A " 


" X2 


1 >A " 


** X 2 


' x* - 



A paper sleeve should then be slid over each wire of a pair. The 
pairs in paper-covered cables are usually colored red and white, and 
as a matter of convenience the paper sleeve should be slid over the 
red wire on one end of the cable, and the white wire on the other. 



822 AMERICAN TELEPHONE PRACTICE. 

The insulation of two wires of the pair should be removed and the 
wires so cut that they will overlap about four inches. Each wire 
should be stripped of its insulation for a length of about one and one- 
eighth inch, care being taken that the wire is not nicked in this pro- 
cess. The two ends of the white wires to be joined should then be 
twisted together, the twist extending through the entire length of 
the bare wire and so as to include about one-half inch of the in- 
sulation. 

The details of the method of splicing the two wires together is 
shown in the left-hand portion of Fig. 600. The paper sleeve is to 
be then slid over the sleeve as shown in the right-hand portion of 
Fig. 600. The reason for including a portion of the insulation in 
the twist is merely to keep the insulation of the wire from sliding 
away from the joint when the paper sleeve is pushed over it. In 
no case should the joint be soldered. 




FIG. 602.— CABLE SPLICING. 

The two ends of the cable with a single pair thus connected is 
shown in Fig. 601. 

The joint in the wires should be so spaced that the joints in a 
pair will not be directly opposite each other, and the joints in 
the various pairs will be approximately evenly distributed 
throughout the entire length of the splice. After all wires are 
spliced hot paraffin should be poured over them, a ladle being 
used to dip the paraffin from the melting pot and pour it over the 
splice. The pot should be placed under the splice so that the sur- 
plus paraffin will drain back into it. This operation of boiling out 
should extend from the ends of the splices towards the middle of 
it and should be continued until every trace of moisture has 
been driven off as indicated by the absence of bubbles. The 
splice should then be thoroughly taped with a strip of muslin 



AERIAL CABLE CONSTRUCTION. 



823 



about one inch wide, as shown in Fig. 602. Some may prefer 
to omit this wrapping of tape. Where it is used, however, the 
splice should again be boiled out after it is in place, after which the 
lead sleeve should immediately be drawn in place as shown in 
Fig. 603. If the splice has been properly made the lead sleeve 




FIG. 603.-CABLE SPLICING. 



will lap the cable sheath* about two inches at each end. The lead 
tube should then be pressed down into close contact with the cable 
sheath at each end, and a wiped joint made carefully about each end. 




FIG. 604.— CABLE SPLICING. 



Figure 604 shows the appearance of the finished joint and also the 
method used in making the wiped joint. 

Where a cable terminates, means must be provided for distribut- 
ing its various wires and connecting them to the wires which are to 
form connections of the same circuits. Means must also be pro- 




824 



AERIAL CABLE CONSTRUCTION. 825 

vided for protecting the cable core so as to prevent moisture from 
entering. The same is true at intermediate points on a cable 
wherever it is necessary to take out some of the wires making them 
available for use at such points. 

There are two general methods of terminating or tapping cables, 
one employing what is known as a pothead or flexible terminal, 
and the other employing what are called box terminals. Potheads 
or flexible terminals are made by joining rubber-covered wires to 
the cable and hermetically sealing the metal tube, usually made of 
lead, to the cable sheath in such manner as to enclose all the joints 
between the paper insulated cable and the rubber insulated wires. 
This tube is then filled with a melted insulating compound which on 
cooling effectually seals the cable, the rubber-covered wires pro- 
jecting beyond the end. 

Box terminals consist usually of iron boxes provided at the lower 
ends with thimbles of brass to which the cable sheath may be 
soldered and through which the wires of the cable may be led to 
conveniently arranged terminals within the boxes. To these ter- 
minals the wires of the cable are soldered, and access is given to the 
conductors from the outside of the box by the fact that the ter- 
minals project through the walls of the box to the outside, they be- 
ing insulated from the metal portion of the box by hard rubber bush- 
ings. After the completion of the connections within the box, 
the latter is hermetically closed by a lid which screws in place, a 
rubber gasket serving to prevent the entrance of moisture. 

Potheads are perhaps, all things considered, the most reliable and 
most economical method of terminating cables. A pothead, how- 
ever, to be effective, should be made with the greatest care, and the 
following are specifications governing their construction used by 
one of the various operating companies in this country. 

POTHEAD TERMINALS. 

Materials. 

Lead sleeves, of unalloyed lead, 1-8 inch thick, of the following 

dimensions. 

For 150 pair cable, length 26 inches, inside diameter 3 X X inches 
" 100 " " " 24 " 3 



20 
25 " 20 



or : 



Drift out the sleeve for one-half its length until its diameter is in- 
creased one-quarter of an inch. 



826 AMERICAN TELEPHONE PRACTICE. 

Okonite Wire; twisted pair, red and black No. 20 B. & S. gauge, 
3-32-inch insulation, without braid or outside covering. 

Okonite Tape; f-inch wide. 

Paper Sleeves; all moisture driven off by dry heat just before 
using. 

Brass Tubing; thin, -J-inch in diameter, length, 2.\ inches less than 
that of lead sleeves. 

Heavy Cotton Twine, or wicking. 

Wiping Solder; containing 43 per cent. tin. 

Sealing Compound; of standard type as furnished for this pur- 
pose. Do not mix the compound with other materials. 

Directions. 

Remove the cable sheath for fifteen inches for all sizes except 15a 
pair cable, when the distance should be seventeen inches. Slip 
the lead sleeve over the cable, drifted end last, and splice the cable 
wires to the okonite wires in the usual manner, joining the colored 
wire of the cable to the red okonite wire of each pair. Cover each 
splice with a sleeve, and keep the splices within a limit of thirteen 
inches from the end of the cable sheath in all cases except when 15a 
pair cable is being used, when a distance of fifteen inches is allow- 
able. Remove all pieces of paper or other debris, bind the cable 
wires as they leave the sheath tightly with several layers of twine to 
prevent the compound entering the cable, and tape all the okonite 
wires together for three inches in such a manner that one-half inch 
of the tape wire will be below the surface of the compound. Open 
up the spliced wires as much as possible to allow free spaces be- 
tween, and bind the brass tube with twine alongside the wires with 
the lower end even with the end of the sheath of the cable, binding 
no higher than the wire splices. Draw up the lead sleeve until its 
lower end laps over the cable sheath ij-inch, and wipe it to the 
sheath. Secure the whole in an upright position, warm the lead 
sleeve until it can barely be touched with the hand, place a funnel in 
the brass tube, and slowly pour in the sealing mixture, previously 
heated to 350 degrees F., until it fills the sleeve to within one-half 
inch of the top. Then remove the funnel and allow the compound 
to settle and cool. Test the compound just before using by putting 
in a short piece of okonite wire for two minutes. If the okonite 
wire is not softened so as to readily come off the wire the compound 
is not too hot. Protect the open end and the wires leading there- 
from against the weather. On the next day fill with hot compound 



AERIAL CABLE CONSTRUCTION. 



827 



to make up for the settlement. Three days later do the same if 
necessary. After this, and when thoroughly cold, dress the top of 
the lead sleeve into contact with the okonite tape wrapping, which 
at this point should consist of at least four layers. Place a cross on 
the outside of the lead sleeve at a point opposite the upper end of 
the brass tube. Do not boil out the cable end with paraffin. If 
dampness enters, the cable should not be used until this defect is 




FIG. 606.-BOX HEAD. 



remedied, or the part cut away. Under no circumstances may par- 
affin be used on okonite ends. 

The completed pothead as made above is shown in sectional view 
in Fig. 605. Sometimes the brass tube is omitted and the seal- 
ing compound is poured directly into the top of the pothead. The 
formation of air bubbles and the incomplete sealing of the cable are 
very much more likely to ensue when this method is employed. 
When the brass tube is used the sealing" compound is really intro- 



82S AMERICAN TELEPHOXE PRACTICE. 

duced at the bottom of the splice, the compound gradually coming 
up from below and forcing all air out before it. 

A typical iron box head is shown in Fig. 606, in which various 
wires are shown extending each from the cable entering the tube at 
the lower portion of the box to one of the terminals within. 

There is an almost endless variety of box terminals adapted to be 
secured to the pole in various ways, and some of these are very re- 
liable, but they are usually more expensive than the pothead. 

The use of lime in hermetically sealed box terminals has been sub- 
jected to a good deal of discussion. The fact that the box terminal 
is sometimes open to perhaps moist air always tends to lower the in- 
sulation of the cable on account of the entrance of moisture. A 
little unslacked lime introduced into the cable head will serve to 
absorb what moisture is in the cable end, or within the terminal itself, 
and thus keep the insulation high. There are other such absorbent 
materials, but most of them are subject to one or more disadvantages, 
some of them being in themselves of a corrosive nature and others 
tending in themselves to lower the insulation. Lime thus used will 
actually raise the insulation where a small amount of moisture has 
been present, and will keep it high, and seems to offer no deleterious 
action upon the conductors or the insulation even after the lime is 
so fully slacked as to crumble in the bottom of the box. 

In modern overhead cable work it is the practice to distribute the 
wires in a cable so as to make a single pair of wires available for use 
at more than one point. To do this single pairs of wires within the 
cable are brought out at intermediate points within the cable, the 
same pair extending on to the end of the cable, or to several other 
intermediate points. This is called multiple distribution. 

In a telephone system, the wire plant is the part needing the 
most careful consideration and best judgment in view of the particu- 
lar condition of the territory to be served. What part of the system 
should be underground, what part in aerial cable, and what part in 
open wire, constitute important fundamental elements; but the dis- 
tribution of the lines from the cables may be done in many ways. 
Multiple cable distribution is a method of line construction in which 
the cable pairs of a system are available at more than one point; the 
object of forming such a system of distribution is to provide for the 
constant and unforseeable changes in the location of substations, 
and for the inevitable errors in estimating growth. In making the 
development of a telephone system, however, the best that can be 
done in the sparsely developed regions is to make a fair assumption 





FTC. 607.— MULTIPLE CAB] E TAT 
829 



830 



AMERICAX TELEPHOXE PRACTICE. 



as to where the various densities of development will occur; and at 
the best this assumption can only be generally correct. It will varv 
in minor ways, even if the broad suppositions proved to be accurate. 
Multiple cable distribution is intended to provide a flexibility which 



S^SE^ 




FIG. 608.— DETAILS OF WIRE JOINT IN Y SPLICE. 

will offset these inaccuracies of assumption; and that system of 
multiple distribution is best which requires the smallest number of 
cable wires to serve a constantly increasing number of subscribers. 

The method of bringing out a certain number of pairs from a 
cable consists in making a "Y" splice at the required point, a cable 
of the required number of pairs that it is desired to bring out being 





CABLE TERMINAL CAN. 



tapped on the main cable, which may be of, say, 50 pairs, the joint 
being made on a lead sleeve with wiped joints after the fashion of 
the splice already described. One method of taking out multiple 
taps consists in the use of regular dry core paper cable for the tap 



AERIAL CABLE CONSTRUCTION. 



851 



cable which is then terminated at its upper end in a pothead as 
already described. A modification of this method consists in plac- 
ing the required number of rubber-covered wires of No. 19 or No. 
20 gauge in a lead tube, such as to very loosely hold the wires, and 
filling this tube with a compound of the same nature as that used 
in pothead work. This pipe cable is tapped on to the main cable 
with an ordinary "Y" splice, and moisture is prevented from entering 
the main cable by the sealing compound in the lead pipe. No other 
seal is then necessary for the cable. In Fig. 607 such taps are 




Sections 

Lead 
wrapjrin 



FIG. 610.— DETAILS OF CABLE TERMINAL. 



shown, these being used by Mr. L. W. Stanton in some of his recent 
work. 

The details of the wire joint used in making "Y" splices are 
shown in Fig. 608. 

Where the cable is terminated by means of a pothead, or some 
form of box terminal, some housing must be provided for out-of- 
door terminals for the conductors to which the external circuits are 
to be joined.. Sometimes this housing assumes the form of a cylin- 
drical can of galvanized iron, as in the case shown in Fig. 607, and 
is better illustrated for a large terminal in Fig. 609. Whore a 
pothead is used the general practice is to support it in an upright 
position on the pole, having its upper end extending into a wooden 
cable box containing conductor strips, lo which the wires leading 
from the pothead and those leading to the outside circuits may be 



832 



AMERICAN TELEPHONE PRACTICE. 



joined. This construction is applied to a multiple tap as shown in 
detail in Fig. 610, this being the standard construction of one of 
the large Bell companies. 

In the case of Fig. 610 the wires leading to the subscribers' 
stations are taken off on insulators attached to cross arms, while in 
other cases what are called distributing rings or circle tops, of which 
one is shown in Fig. 611, are provided. 

Methods of distribution from aerial cables to subscribers' prem- 



*z±^ 




FIG. 611.— DISTRIBUTING RING OR "CIRCLE TOP. 



ises differ greatly. The old method was to terminate the cable on 
a pole and then to continue the conductors as bare wires on cross 
arms until the destination of a pair was reached, after which "drop" 
wires were run from the pole nearest the subscriber's premises to 
his house. This bare wire distribution has largely gone out of use 
and plants have been constructed in this country in which hardly a 
foot of bare wire was used. This latter practice, however, is thought 
to be carrying the cable idea to an extreme. At present the best 



AERIAL CABLE CONSTRUCTION. 



833 



practice after terminating the cable by any of the methods described, 
is to run a No. 18 B. & S. rubber-covered braided wire from the 
terminal in the cable box to the distributing insulator on the same 
pole, and then drop off from its insulator with the drop wire to the 
subscribers' premises. Drop wires differ largely, both bare and in- 
sulating wire being used. Drop wire should be No. 14 hard drawn 




FIG. 612.— TEN-PAIR TERMINAL DISTRIBUTING TO RESIDENCES. 



copper if bare. If both are insulated, they should be not smaller 
than No. 16 rubber-covered and braided, unless they are twisted, in 
which case they may be as small as No. 18. Frequently one bare 
and one insulated wire is used, these being run separate in practi- 
cally the same manner as two bare would be run. In this case 
both wires should be of No. 14 hard drawn copper. 

In Figure 612 is well shown the methods of extending drop wires 
53 



834 



AMERICAN TELEPHONE PRACTICE. 



from a multiple tap terminated in a can to the various subscribers' 
houses. Figure 613 shows a can at the top of a pole feeding to a 



3 



> 




FIG. 



613.— TWENTY-FIVE-PAIR TERMINAL DISTRIBUTING TO 
BUSINESS BLOCK. 



distribution ring from which the various drop wires are led. These 
cuts were loaned by Mr. L. W. Stanton, whose construction work 
they represent. 



CHAPTER XLI. 
UNDERGROUND CABLE CONSTRUCTION. 

It is now settled practice to place all telephone wires underground 
in the business centres of large cities, and even to do so in the cen- 
tral parts of smaller cities. The primary requisite for this construc- 
tion is that a suitable conduit shall be provided in which the con- 
ductors may be laid. It is usually necessary to provide conduits 
having a suitable number of ducts to meet the requirements for 
future as well as immediate use, and much judgment should be 
exercised in this respect in planning the system. Suitable 
openings are provided for the conduits at frequent intervals, 
these being in the form of manholes, from which sections of the 
cables may be drawn into the ducts and withdrawn when occasion 
requires, for repairs. The principal requirements of a good con- 
duit may be outlined as follows: 

The material of which the conduit is made must be durable, 
and this implies that it must be absolutely proof against decay or 
corrosion due to moisture, dry rot, gases, or the liquids present 
in the soil. It should, moreover, be fire-proof if possible, al- 
though this is a minor consideration. 

The conduit should possess both tensile, shearing and crush- 
ing strength. Severe vertical strains are frequently imposed upon 
subway structures, due to the removal of the support from be- 
neath them, caused by excavations in the streets or by the 
settling of the ground. Side strains are not so likely to occur, 
and their effects are usually slight; therefore, it follows that 
the conduit should be, if possible, strongest in a vertical direc- 
tion. If the stress imposed upon the structure is such as to cause 
a fracture or undue settling, the alignment of the ducts is thereby 
destroyed, which may interfere with the drawing in or with- 
drawal of cables. Moreover, the grade of the duct is destroyed, 
so that the proper drainage cannot be effected. The ducts be- 
tween the manholes should be straight, if possible, and where curves 
are necessary they should be very gradual and present no sharp cor- 
ners which would interfere with the drawing in or seriously abrade 
the cable sheath. Slight turns in conduits are frequently made 



836 AMERICAN TELEPHONE PRACTICE. 

by joining together short straight sections, but where the nature 
of the conduit used permits it, it is better to form all bends of 
curved sections. It is desirable that the structure should be com- 
posed of insulating material and be moisture-proof. No depend- 
ence, however, for insulation of the conductors themselves must 
be placed on the conduits, as the cables must in all cases provide 
the means for keeping the conductors thoroughly insulated and 
free from moisture, even under the most adverse circumstances. 
No conduit system has yet been constructed which has been kept 
dry, and to do so means to seal each end of each duct at each man- 
hole. As yet, it has not been found absolutely necessary to get such 
dryness. 

It is very essential that the conduit must contain no chemical 
agents capable of exerting a deleterious effect on the cable 
sheath. As an example of this may be mentioned certain forms 
of wooden conduits, which in the process of decay liberate acetic 
acid, which in a short time totally destroys the cable sheath, 
changing it to lead acetate. This difficulty has been experienced 
with some forms of creosoted wood conduit, but in the later 
products in this line this difficulty is said to have been completely 
removed by the use of a better grade of creosote oil and improved 
methods. 

Economy of space is often an important item in the selection of 
conduit to be used, and under crowded conditions that conduit which 
will place a given number of ducts within the smallest space is the 
most desirable, other things being equal. 

The earliest form of conduit used in this country was the open-box 
conduit, which consisted merely in a trough made of inch-and-a- 
half or two-inch lumber and of sufficient size to accommodate 
enough cables to meet the existing demands, as well as the future 
growth of the system. These troughs were laid in a trench, the bot- 
tom of which was properly graded, the sections of the trough being 
about fifteen feet in length and butt-jointed — that is, laid together 
end to end. The joints were held in line by boards nailed on the 
outside and overlapping each end about a foot. The cable was 
laid in these troughs by driving the reel containing it slowly along- 
side of the trench, the cable being carefully laid as it was unwound 
from the reel. After all the cables were in place the trough was 
rilled with hot pitch, when the cover was nailed in position and the 
trench refilled. This is probably the simplest form of underground 
cable construction, with the exception of a method, infrequently 



UNDERGROUND CABLE CONSTRUCTION. 



837 



practiced, of laying the cable directly in the ground without any 
conduit whatever. 

The cheapest and simplest form of conduit which permits the 
drawing in or withdrawal of the cables is that composed of creo- 
soted wood tubes, or "pump logs," as they are commonly and 
appropriately termed. These are usually made in eight-foot lengths, 
having a square external section 4^x4^ inches, with a 3-inch bore. 
A tenon joint one and one-half inches long is used for securing 
proper alignment of the joint. Several views of this tube are shown 
in Fig. 614. 

The wood is usually treated with creosote or dead oil of coal 
tar in the following manner: The lumber is laid on cars and run 
into a large steel cylinder six feet in diameter, which is closed by a 
heavy iron door. It is first subjected to live steam at a temperature 
of 250 F. until the timber is heated through and through, the pur- 
pose of this being to coagulate the albumen in the sap. A vacuum 




FIG. 614.-"PUMP LOG" CONDUIT. 



pump is next applied to the tank, exhausting all air and steam, the 
pump maintaining a vacuum of about twenty-six inches. This 
evaporates practically all of the sap and water from the wood, thus 
seasoning the timber. The next step in the process is to pump creo- 
sote oil previously heated to a temperature of ioo° to 125 F. into 
the tank until it is full. This is then placed under a pressure of 
about eighty pounds per square inch and the amount of creosote 
which is forced in after the filling of the tank is carefully measured, 
this being the amount that is taken up by the pores of the wood. 
Specifications for the treatment require that from eight to twenty 
pounds of the oil shall be absorbed by each cubic foot of timber. 
Twelve or fifteen pounds is the average amount required for elec- 
trical purposes. As has been stated before, much trouble has ex- 
isted owing to the liberation of acetic acid from conduit treated with 
creosote. It is claimed, however, that by using a proper quality of 
creosote oil, and by using the method of impregnation just de- 



83,^ 



AMERICAN TELEPHONE PRACTICE. 



scribed, that this trouble has been entirely eliminated. The life of 
creosoted wood conduit is, to say the least, problematical, but there 
seems to be good reason to believe that when properly treated and 
laid it will last an ordinary lifetime, if not longer. 

In laying this conduit the trench is dug, and after its bottom 
is properly graded so as to have a gradual slope either from an in- 
termediate point toward both man-holes or an uninterrupted slope 




FIG. 615.— SINGLE AND MULTIPLE-DUCT CONDUIT. 



from one man-hole to the other, a creosoted wood plank two inches 
thick is laid as a foundation. The ducts are then laid on this plank 
side by side and in as many different layers as are necessary to give 
the required number. They should be so laid that the separate 
ducts break joint in order to give strength to the entire structure. 
Over the upper layer is then laid another creosoted wood plank two 
inches thick, after which the trench is filled in with earth. The 
great point in favor of this conduit is its cheapness, this being greatly 



UNDERGROUND CABLE CONSTRUCTION. 



839 



enhanced by the fact that no concrete is employed for a foundation. 

Conduits of clay or terra-cotta, burned hard and with vitrified 
surface, are being extensively used and are giving unqualified satis- 
faction. These are made up in a number of forms which may be 
divided into two classes, namely, multiple duct and single duct. 
The multiple duct conduit is made up in a variety of ways, some 
of which are shown in cross-section in Fig. 615, loaned by the 
H. B. Camp Company. For telephone purposes, the duct opening 
is either round and 2,2 to 4 inches in diameter, or square with round 
corners and of the same dimensions both ways. 

The earliest form of clay duct, which was used at all, generally 
was one having a square outer form in section, about 12 by 12 inches, 




FIG. 616.— SINGLE AND QUADRUPLE TILE. 



walls of one inch, and a horizontal shelf across the middle, one inch 
thick. This divided the pipe into two ducts, one above the other, each 
about zlJ inches high by 10 wide. Into each duct were drawn three 
cables, two side by side, and one on top of these two. Economy of 
duct space was gained at a sacrifice of safety, as it was found im- 
possible to draw out one of three, if it became necessary, without 
tearing ihe sheath off in doing so. 

Single-duct tile then came into existence, and still later the mul- 
tiple-duct type, but these latter, as Figs. 615 and 616 show, had one 
duct opening for each cable. Fig. 616 is loaned by Field Clay Con- 
duit Company. 

The single-duct class of tiles possesses some advantages over the 
multiple-duct tiles, chief among which are the greater flexibility 



840 



AMERICAN TELEPHOXE PRACTICE. 



and the increased ease of handling. The form shown in Fig. 617 
has come into very wide use and has proven its adaptability to meet 
almost any conditions that may arise. These tiles are 4! inches 
square by 18 inches long, and have a 3^-inch bore. By it curves 
are easily made, short curved lengths being provided, or curves of 
long radius may be made with the regular tiles, the lengths being so 
short as to form a practically smooth interior surface. This conduit 
is laid in much the same way as ordinary brick, and in order to 
insure proper alignment a mandrel (shown in lower portion of Fig. 
617), three inches in diameter and about thirty inches long, is laid in 
the duct and pulled along through it by the workmen as each ad- 
ditional section is laid on. The rear end of this mandrel is pro- 
vided with a rubber gasket a little larger than the diameter of the 




FIG. 617.— SINGLE-DUCT CONDUIT AND MANDREL. 



conduit, which effectually smoothes the inner surface and pre- 
vents the formation of lips which might prove injurious to the 
cable sheaths in drawing in. On the front end of the mandrel is 
provided an eye which may be engaged by a hook carried by the 
workmen in order to move it forward. Fig. 618 shows two examples 
of single-duct subway in process of construction. 

In laying vitrified clay tile the process used is as follows: 
The trench is dug to such a depth as to allow at least two feet of 
earth above the top of the entire structure. Some specifications call 
for as great depth as three feet, but this is necessary only where 
there is a probability that new ducts may be added to the conduit 
in the future. The width of the trench should be about six inches in 
excess of the actual width of the number of ducts which are to be 
laid side bv side. In the bottom of the trench is laid a concrete 



UNDERGROUND CABLE CONSTRUCTION. 



841 



foundation to a depth of from three to six inches — the former under 
ordinary circumstances is sufficient. The tiles are then laid in place 
in cement mortar, and as each layer is finished the sides of the trench 
should be filled to the top of that layer with the same concrete as 
that used for the foundation. The space between the tiles in a layer 
and between the layers should be carefully filled with good cement 
mortar mixed thin enough to readily fill the interstices. After the 
required number of layers are in place the top is covered with a 
mass of concrete not less than four inches in thickness. 

The concrete used in this work should be composed of one part of 




FIG. 618.— TWENTY-DUCT SUBWAY. 

hydraulic cement, two parts of clean sharp sand, and five parts of 
broken stone, screened gravel, or broken brick. The size of the broken 
stone or brick or gravel should not be larger than one inch in any 
dimensions. The cement and sand should be thoroughly mixed 
while dry, and then enough water added to form a soft mortar, after 
which the broken stone should be thoroughly mixed in. 

The mortar should be composed of one part of hydraulic cement 
and two parts of clean sharp sand, thoroughly mixed together, and 
then with water as before. It is a matter of greatest importance that 
the ducts should not be moved while the mortar or concrete is setting. 



842 AMERICAN TELEPHONE PRACTICE. 

After the entire subway is laid from one man-hole to the other it 
is advisable to draw through it a scraper, thus removing all projec- 
tions on the inside walls. The ducts may then be washed out with 
a hose, thus removing all grit and leaving a clean, polished tube. 

Another style of conduit is cement lined pipe. This, as usually 
constructed, consists of a wrought-iron pipe of Xo. 26 B. W. G., 
with riveted joints, the rivets being set one and one-half inches apart. 
This pipe is lined with Rosendale cement, the thickness of the lining 
being five-eighths of an inch, and the interior of the lining being 
polished. The standard size of this tube is in eight-foot lengths, 
with a three-inch bore. It is provided with cast-iron ball and socket 
joints at the ends in order to insure proper alignment and to provide 
a certain amount of flexibility in making turns. 

This conduit is laid in concrete in much the same manner as the 
clay pipe, it being common practice to separate the different pipes 
in the layer by about one-half of an inch and the various layers 
themselves by about one inch. While a great deal of such conduit 
is in use, not much of it is now being laid. 

In laying conduit in city streets numerous obstructions are met, 
and must be overcome in the manner best suited to the individual 
case. It frequently becomes necessary to remove the support from 
heavy pipe lines for a considerable distance, as, for instance, when 
such a pipe line lies diagonally across the trench. In all cases 
suitable supports for these pipes or other structures should be pro- 
vided until such time as the trench is again filled. The usual means 
adopted is to place a beam of sufficient strength across the top of 
the trench and support the pipe therefrom by chains or heavy rope.. 
It is frequently necessary in passing an obstruction to fan out the 
pipes in one layer so that they occupy the same level as those of 
another layer. Such a construction, and also a rather crooked piece 
of conduit work, is shown in Fig. 619, where, on account of ob- 
structions in the street, the two layers of two pipes were formed into 
one layer of four until the obstructions were passed. This par- 
ticular obstruction was a sub-cellar extending out under the street. 

The manholes may be built of various forms and dimensions to 
meet existing requirements. In the best construction the founda- 
tion consists of a layer of concrete six inches deep, the concrete 
being mixed as specified for the laying of tiles, with the exception 
that the crushed stone may be considerably coarser. The walls of 
the manhole are then built of good brick-work of suitable thickness 
and well plastered on the outside with cement mortar in order to 



UNDERGROUND CABLE CONSTRUCTION. 



843 



exclude as much dampness as possible. For the ordinary manhole 
an eight-inch wall is sufficiently thick, but in building very large 
underground vaults it sometimes becomes necessary to double or 
treble this thickness. Where these very thick walls are required it 






FIG. 619.-AVOIDING OBSTACLES. 



is good practice to allow about one inch air space between the outer 
course of brick and the inner in order to render the interior as dry 
as possible. A common-sized manhole is five by five by five feet, and 

smaller sizes down to three bv three feet with five feet head room are 



844 AMERICAN TELEPHONE PRACTICE. 

also common. As a rule a manhole should provide at least enough 
room for two men to work in conveniently. Of course, where a 
great number of ducts enter a manhole the size must be increased 
accordingly. 

After the conduits are laid and the manholes finished the next 
step is the drawing in of the cables. In order to accomplish this 
a process called rodding is in most cases first necessary, in order 
that a rope may be stretched through the duct, which is afterwards 
to be used for drawing in the cable itself. For this purpose a large 
number of wooden rods about three-fourths of an inch in diameter 
and four feet long, and equipped with screw or bayonet joints at 
each end, so that they may readily join together, are necessary. A 
man stationed in one of the manholes inserts one rod into the duct, 
and, after joining another rod to it, pushes this also into the duct. 
Successive rods are joined and pushed through until finally the 
first rod reaches the next manhole. A rope is then attached to one 
end of the series of rods, which is then pulled through, unjointing the 
rods as they are taken out of the duct. Where the ducts are smooth 
and comparatively straight this process may be simplified by using 
a continuous steel wire about one-fourth of an inch in diameter in 
place of the rods. It is a good plan to attach to the forward end of 
this wire a lead ball, which will facilitate it in riding over obstruc- 
tions. 

The cable reel is then placed near one of the manholes in such 
manner that the cable will pay out from the top of the reel instead 
of from the bottom. The end of the cable is then attached to the 
rope and started into the duct. In the distant manhole the rope is 
led over one or more sheaves suitably arranged on upright beams 
placed within the manhole to a capstan or other form of windlass 
by which the cable may be slowly drawn through the duct. A fun- 
nel-shaped shield should be placed at the mouth of the duct into 
which the cable is being fed for protecting the shield against the 
sharp corners at the entrance. This shield, however, is not a suf- 
ficient protection for the cable, and one or more men should be 
stationed in the manhole for guiding the cable into the duct. The 
best way to attach the rope to the end is by means of clamps es- 
pecially provided by the cable companies for this purpose. How- 
ever, if these are not used, a secure grip may be had upon the cable 
end by winding several strands of stout iron wire in opposite direc- 
tions about the cable sheath for a distance of two feet from its end. 
An eye may be formed in this wire opposite the cable end to which 



UNDERGROUND CABLE CONSTRUCTION. 



845 



the rope may be attached. Particular attention should be paid to 
the sealing of the cable end before it is drawn into the duct, as ducts 
are always moist, due to sweating of the interior walls, or to leaks. 

Where a large amount of cable is to be drawn in the method 
shown Fig. 620 may be employed. Instead of the hand-operated 
winch or windlass a three and one-half horsepower horizontal en- 
gine and capstan mounted on a low wagon is used. By suitable 
gearing the engine causes the capstan to revolve slowly. This 
method was used in the recent extensive underground construction 
work in St. Louis by the Bell Telephone Company, of Missouri. 
With this contrivance a speed of twenty-five feet of cable per min- 
ute is easily attained without in any way damaging the cable and the 
remarkably short time in which the enormous amount of cable in- 




$mM&&. 



FIG. 620.-DRAWING IN BY STEAM POWER. 



stalled by that company was drawn in testifies further to the practi- 
cal value of this scheme. 

Mr. C. L. Zahm has used with success in cable-drawing the form 
of winch shown in Fig. 621. This is an electric-motor arrange- 
ment, the motor being wound to operate on a voltage of 600 or 
thereabouts, and geared down to the drum. Power is taken from 
any convenient circuit carrying direct current at 500 to 650 volts, 
and if it be a trolley wire, the connections are very simply made by 
means of a portable trolley pole and flexible wire. The wheels on 
the whole outfit enable it to be taken from place to place with ease. 
Another convenience used by Mr. Zahm is shown in Fig. 622. This 
reel-truck has strong spider wheels with ample width of tire, and has 
an extended shaft enabling a clevis tongue to be attached. A 
team of horses can then draw the reel and truck. When set for 
paying off cable the reel turns on the shaft. » 

Cables, in passing through manholes, should be laid around the 
side of the manhole and supported on hooks provided for that pur- 



•846 AMERICAN TELEPHONE PRACTICE. 

pose. Shields formed of sheet lead or of heavy felt should be placed 
under each cable just at the point where it emerges from the duct, 
in order to prevent injury of the sheath at that point. Workmen 
should be cautioned against needlessly bending cables while working 
in ducts, and the use of the cables in place of ladders for climbing in 
and out of the manholes should be strictly prohibited. Slack should 
be left in the manhole, in order to allow room for subsequent splicing 
when necessary. 

Trouble is frequently experienced, due to the presence of gas in 



FIG. 621.— ELECTRIC WINCH. 

the manhole, due to leakage through the earth from gas mains, and 
care should always be exercised before striking a match or taking 
a torch into a manhole, to make sure that all gas has been removed. 
There are several methods of doing this, one of which is to pump 
the gas out with an inverted umbrella made specially for the pur- 
pose. The umbrella is lowered into the manhole while closed and 
then suddenly withdrawn, this opening the umbrella and lifting out 
the gas. Another way of clearing manholes from gas is to place 
a cloth screen above the manhole and on the side opposite to that 



UNDERGROUND CABLE CONSTRUCTION. 



847 



from which the wind is blowing. The wind on striking the screen 
is deflected downward, thus causing an eddy which removes the 
gas from the manhole. Very serious explosions have been caused 
by the collection of gas in manholes, which becomes ignited either 
by an electric spark or by the torch of a workman. 

One of the most serious difficulties in connection with under- 
ground cable work is that brought about by electrolysis, due to 
the action of stray earth currents, usually due to the ground return 
of electric railways. It is found that the electrolysis occurs at points 




FIG. 622.— REEL TRUCK. 



where a current flowing along the cable sheath leaves the sheath 
and enters the ground. At this point oxygen is liberated, which, 
with the chemicals in the earth, rapidly corrodes the lead sheath. Of 
course, the construction of conduits, composed of insulating mate- 
rial, will do something towards the alleviation of this trouble. 

Frequent tests should be made, however, on all cable systems to 
determine the polarity of the cable sheaths with respect to sur- 
rounding conductors. The tests for this purpose may be made as 
follows: Two brass rods about six feet long should be provided, each 



848 AMERICAN TELEPHONE PRACTICE. 

having a steel contact at one end. Between these two rods should 
be connected by flexible wires a portable voltmeter — one reading to 
five volts will usually be found most suitable. The test should be 
made at the manholes, these being the most available points for 
reaching the cable. One of the steel contact points should then be 
placed in firm contact with the cable sheath and the other into con- 
tact with any water or gas pipes which run through the manhole, 
and in each case the voltage should be noted, not only in amount 
but in direction. Reading should also be taken between the cable 
sheaths and the rails of adjacent electric railroads, and to whatever 
underground structures exist in the immediate vicinity. It is evi- 
dent that where the cable sheaths are negative to the surrounding 
conductors no danger will exist, as this would indicate that the cur- 
rent tended to flow from the other conductors to the sheath. If, 
however, the cable is found positive to the surrounding conductors, 
the matter should be carefully followed up by taking readings in 
successive manholes. By these means the maximum danger point 
can be located, it being, as a rule, the point at which the maximum 
positive difference of potential exists. At this point the cable sheath 
should be securely bonded by a heavy conductor to the water or gas 
pipe or to other metallic structures that are in the vicinity. These 
bonds serve to allow the current to flow from the cable sheath to 
the other conductors, instead of forcing it to find circuit through 
the ground or through the walls of the conduit. In some cases 
the only remedy has been to run separate return circuits from 
the maximum danger points on a cable directly to the power-house 
from which the troublesome current emanates. 

All of the cable sheaths entering a manhole should be bonded to- 
gether, the usual method of doing this being to brighten the surface 
of the lead sheaths and to bend a No. 10 B. & S. copper wire around 
each sheath, afterwards soldering the connection. This assures 
the fact that all of the cable sheaths will be at an equal potential and 
that whatever bonds are run for the protection of one sheath will 
afford protection for all. The method of bonding to a gas pipe 
usually adopted is as follows: The surface of the pipe is brightened 
for a space of about three by eight inches with a coarse file. This 
surface is then heated by a torch and tinned with ordinary solder. 
A copper plate about three by seven inches previously tinned is then 
soldered to the gas pipe, after which the bond wire leading from the 
cable is wound into a flat coil and soldered to a copper plate. In 
bonding to a water pipe it is impossible to heat the pipe sufficiently 



UNDERGROUND CABLE CONSTRUCTION. 



849 



to make it take solder on account of the water flowing within. The 
method to be followed is to provide a heavy wrought-iron U-shaped 
band adapted to fit snugly around the pipe. The ends of this band 
are screw-threaded and pass through a yoke-piece bent to fit the 
upper portion of the pipe. This yoke-piece is then firmly screwed 
in place by nuts, the surface of the pipe and the interior of the iron 
clamp having previously been thoroughly brightened. The bond 



COST in CENTS PER DUCT FOOT 
13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 












6 t *? *.-"' 


^ ZJ? *** 


e -7- J& y ^ 


z ,w. s 


10 /L fiC -** 


jT <$- S 


1? it £/ y 


>• \M 1 / 


a u St M- z ± ± 


> u % is is 


n K TZ # # 


,2 6 / J 12 


10 18 3 I? ?A 


h I ii S 


5 Z o t ^ # 


B <° Jt ZK £2 


g 22 3L Jl la 


8 t -S $ 


t- 24 I 3 +1 


£ £4 jfc a Si 


'0 ?fi o 5/ £/ 


R " It - % 


h ? o j i ft 


g za > ir it 


P 30 h. ± t 


b * ■ S . J 8 


2 32 I - K 3 


PV z Ji_ l _£■ 


y 34 i . s ^ . 






r 36 : : : 


38 








<d? 






*W 


4fi 




dfl 





FIG. 623.— CURVE OF CONDUIT COSTS. 



wire may then be soldered to this yoke-piece and the whole device 
smeared with asphalt paint. 

A form of duct material other than that made of clay is made of 
paper or similar fibre, saturated while making, or after making, with 
some asphaltic or bituminous compound. The form of a complete 
duct of this kind is round, and of cylindrical bore. The thickness of 
the walls does not need to be great, and there is next to no danger 

54 



850 AMERICAN TELEPHONE PRACTICE. 

of breakage. It is claimed that there is an advantage in the fact 
that this form of material will not absorb moisture, and that there 
will thereby be very much less danger of electrolytic corrosion. It 
is certainly true that there is much less weight per foot of duct, and 
a less loss by breakage. Whether the material is a satisfactory one 
for a structure that ought to be expected to last fifty years or more 
is a question which it is not attempted to answer here. It is dif- 
ficult to see, however, why it is not. 

In the matter of estimating the cost of an underground system of 
distribution of telephone wires, there is no royal road; the costs of 
the material and labor for the varions parts of even one system will 
var.y so much that there is only one accurate way, and that is to lay 
out the routes and figure the cost of each, adding the results. For 
purposes of preliminary estimates, however, it is often necessary to 
approximate final results, and to do this one may not always need to 
know all the details of the final plan. Mr. C. J. Field has con- 
tributed a valuable compilation of data in giving engineers a graphic 
table of approximate costs. The curve in Fig. 623 gives such costs 
based on a duct material price of five cents per foot. To use the 
curve in compiling an estimate, tabulate the runs of conduit under 
their various sizes and character of pavement, and look out in the 
curve the cost per duct foot for each. Multiply the cost thus found 
by the number of ducts in that trench, and by its length ; do the same 
for all the trenches, and add the result. The sum will be the cost 
of the system. If the cost of duct material on the ground is less, or 
more than five cents, the curve must be modified accordingly, but 
need not be necessarily redrawn. 



CHAPTER XLII. 

TESTING. 

Tests of telephone lines, whether of bare wire on poles or of over- 
head or underground cables, may be divided' into two general 
classes : 

First : Those which are for the determination of the existence 
of certain conditions, without the necessity of measuring quanti- 
tatively the extent to which those conditions exist ; in other words, 
rough tests for the determination of grounds, crosses, or breaks, 
usually made with instruments such as the magneto bell, tele- 
phone receiver and battery, and a few other such simple but often 
in the hands of an experienced person most effective instruments. 

Second: Those for not only determining the existence of cer- 
tain conditions, but also for their quantitative measurements. These 
require the use of different and more intricate instruments, and in 
many cases the operator must be possessed of a fair degree of 
mathematical training combined with an ingenuity for meeting and 
mastering unusual problems that arise under different conditions. 

The magneto testing set is the most important instrument in 
making tests under the first class. Such an instrument usually 
consists of a powerful magneto generator, so wound as to enable 
it to ring its own bell through a resistance of from 25,000 to 75,000 
ohms. A powerful magneto telephone is carried on the outside of 
the case in suitable clips, and may be switched in circuit alternately 
with the generator by a small hand switch. This magneto telephone 
serves as both transmitter and receiver, and enables the lineman or 
other party to communicate from a pole top or man-hole with any 
other party on the circuit. Frequently these sets are made to include 
microphone transmitter and battery ; but, inasmuch as the instrument 
is seldom if ever used to talk over very long circuits, the extra weight 
of these is considered in most cases undesirable. A small, inex- 
pensive galvanoscope or current detector will also prove very con- 
venient. 

In testing for a ground on a wire, whether it be in a cable or 
bare, and on poles, make sure that the far end of the line is open 
and then connects one terminal of the magneto bell to the near 

S51 



852 AMERICAN TELEPHONE PRACTICE. 

end of the line and ground the other terminal. The ringing of 
the bell would seem to indicate that the circuit was complete and the 
line grounded in this case, but this is not always true, and this test 
must therefore be relied on only with caution. The static capacity 
of a long line or of a comparatively short length of cable will often 
allow enough current to pass to and from the line in charging and 
discharging to ring the magneto bell. 

For testing out local work where there is no room for this ca- 
pacity effect, the magneto bell is invaluable. 

A more reliable means of making tests for grounds or crosses 
is to connect the current detector in series with several cells of 
battery and to ground one terminal. Then with the other ter- 
minal make contact with the near end of the line. A kick of the 
needle will take place in any event on closing the circuit, due to the 
current flowing to charge the line, but a permanent deflection will 
indicate a ground. 

In testing for a cross, as, for instance, with some other wire in 
the line or cable, one terminal of the magneto bell or the galvano- 
scope and batteries should be connected to the wire under test and 
the other to all the other wires in the same lead, for which purpose 
they are bunched. In case it is not convenient to bunch them, how- 
ever, the test may be made between the suspected line and each of 
the others in succession. 

Another and perhaps still more simple method for determining 
a cross or ground is one described in Roebling's pamphlet on Tele- 
phone Cables, and illustrated in Fig. 624, as applied to the testing 
of a cablebefore it has been unreeled. 

N represents the near end, and F the far end of the wire being 
tested. B is a battery, of about three cells. T is an ordinary tele- 
phone receiver. The wire, N F , is carefully separated from all the 
others at each end. 

At the near end all the wires are stripped of insulation and, ex- 
cept the one under test, are connected together and also with the 
sheath. The wire, C, connects the sheath to one side of battery, B, 
and the other side of battery is connected to one side of telephone 
receiver, T. The testing man rapidly taps with the wire, N F, the 
unoccupied binding post of the receiver, T. The first tap will pro- 
duce in the receiver a distinct click, and if the cable is long there may 
possibly occur a second faint click, but if the wire, A r F, is perfectly 
insulated no more sound in the telephone will follow the tapping. 
If, however, the wire, A T F, is crossed with any wire in the cable, or 



TESTING. 



853 



with the sheath, every tap will be followed by a distinct click, and if 
there is moisture in the paper, making a partial connection, clicking 
sounds will occur, which are loud or faint, according to the amount 
of moisture present. 

The philosophy of this method of testing is very simple, and serves 
to make the operation more readily understood. 

When the wire, N F, is first connected to the battery it becomes 
charged. During the process of charging a current flows into the 
wire and passes through the coil of the receiver and causes the click. 
If the wire is well insulated, the second tap, immediately following, 
finds it charged, or nearly so, and there is, therefore, no click, or a 
very faint one. If, on the contrary, the wire under test is crossed with 
any of the other wires, or imperfectly insulated from them, or from 
the sheath, the wire will immediately discharge itself through the 
cross to the other wires and the sheath, and there will be a flow of 




FIG. 624.-RECEIVER TEST FOR CROSSES AND GROUNDS. 



current at every tap, and consequently a continuous clicking. If a 
conductor in a perfectly insulated cable is very long, two or three 
taps or a long first contact may be necessary to charge it completely. 

If the cable is in place or if it is a bare aerial line that is being 
tested this same method may be used. In case of a new cable it is 
well to test every wire in this manner, and therefore the wire. N F, 
should be put aside and another slipped out of the bunch and tested 
in the same way, and so on until all have been gone over. 

If any of them are found to be in trouble, it is well to carefully 
inspect the exposed ends to be sure they are properly cleared from 
each other and from the sheath. If it is still found to be defective. 
it should be plainly tagged. 



854 



AMERICAN TELEPHONE PRACTICE. 



In the manner just described, twenty-five minutes with two men 
should be ample time for testing one hundred wires, the testing op- 
erator listening and his helper attending to the connection of the 
different wires at N. 

For this test, as well as many others, it is very convenient to 
use a regular operator's receiver and head band, as it will save the 
tester a very tired arm at the end of a long test. As a matter of fact, 
the receiver is little appreciated as a testing instrument. A very 
convenient set is formed by a watch-case receiver and head band, 
and two small-sized cells of dry battery, strapped together so as 
to be carried in the coat pocket. The receiver and battery are con- 
nected in series, the free terminals of the circuit being formed by 
flexible cords about four feet long. These cords should terminate in 




FIG. 625.— CONTINUITY TEST. 



convenient clips, or contact points adapted to make contact with the 
wires to be tested. This arrangement leaves both hands free at all 
times, and is wonderfully sensitive. 

The continuity test, or test for broken wires, may be made with 
the same simple instruments. The wires to be tested should all be 
grounded or connected to a return wire at the far end. At the near 
end, one pole of a magneto bell, or of the battery and galvanoscope, 
or of the receiver, should be connected to ground or the return wire 
and the other terminal connected successively to the terminals of the 
line, which, of course, should all be separated. A ring in the case of 
the magneto, or a permanent deflection of the needle in the case of 
the galvanoscope, or a continuous clicking in the receiver, will indi- 
cate that the wire is continuous. The same precaution as previously 



TESTING. 



855 



pointed out must, however, be observed with the magneto bell. This 
same test for continuity is well illustrated in Fig. 625, in which case 
a vibrating bell instead of the receiver or galvanoscope is used. 

In testing a cable all defective wires should be marked "crossed," 
"grounded," or "broken" at the end at which they are tested. The 
corresponding ends of the tagged wires at the other end of the cable 
should then be found and similarly marked. If there are not the 
requisite number of good wires in a new cable it should be rejected. 

It is often desirable to be able to pick out a certain wire at some 
intermediate point in an open cable, or in a large bunch of insulated 
wires, in order to establish a branch connection. This is easily done 
by the foregoing methods if the cable is to be cut, but frequently 
this is not the case. It may be done without cutting by the following 
simple method : Ground the wire or wires desired at the distant end, 




FIG. 626.— DIAGRAM OF WHEATSTONE BRIDGE. 



being sure that these wires are free from all the others at both ends. 
Then having loosened the bunch of wires at the point at which the 
branch is to be taken off, test each by means of a needle-pointed in- 
strument, connected to ground through a bell or receiver and battery. 
The needle-point can readily pierce the insulation and make good 
contact with the conductor within. A knowledge of this very simple 
test will often save an immense amount of trouble. 

In the second class of test — that is, those requiring quantitative 
measurements — there are three distinct subdivisions, which are as 
follows: Tests for resistance or conductivity, tests for capacity, 
and tests for insulation. Tests for the location of faults in lines 
always depend on the application of one or more of these. 

There are three principal methods of making resistance tests : 
First, by the use of a Wheatstone bridge, which is accurate for all 
resistances except those very large or those very small. Second. 



856 AMERICAN TELEPHONE PRACTICE. 

the fall of potential method, which is largely used for making many 
tests in telephone work. Third, by the use of a sensitive galvano- 
meter in series with a battery. This method is the most accurate for 
the determination of extremely high resistances and is, therefore, of 
great use in measurements of insulation resistance. 

For general resistance measurements the Wheatstone bridge is 
the most suitable, being very accurate and exceedingly simple in 
manipulation. In order to appreciate the possibilities of this instru- 
ment its underlying principles should be understood. In Fig. 626, 
A, B, R, and X represent resistances. G is a galvanometer or in- 
strument for detecting the flow of current. The four resistances are 
connected together as shown, the galvanometer being connected in 
the "bridge" between the junctures of A and R, and B and X. A 
battery, B', is connected between the junctures of A and B, and of 
R and X. Each resistance, A, B, R, and X, forms what is termed 
an arm of the bridge. 

The two fundamental laws upon which the action of the bridge 
is based may be stated as follows : 

1. No current will How between points of equal potential; and 

2. The drop in potential along the various parts of a conductor 
is proportional respectively to the resistances of those parts. 

Referring again to the diagram, it is evident that a current 
from the battery flows to the point, e, where it divides, part flow- 
ing through A R and part through B X, after which they unite 
and pass to the negative pole of the battery. But what of the 
galvanometer? Evidently by Rule 1 the only time at which no 
current will pass through it will be at the time when the points, 
/ and h, are at the same potential. By Rule 2 these points will be 
at the same potential only when A bears the same relation to R as B 
does to X. 
That is 

A : R : : B : X, or, by alternation, 
A_ R^ 
B ~ X' 
A little algebra will render the above evident if not so already. 
Call a the drop of potential between the points e and i, b that 
between e and /, and c that between e and h. 
Then 

b : a : : A : A + R by Rule 2. 

. a A 



TESTING. 857 

Similarly 

_ B 



B+ X 

For a condition of equal potentials at / and h so that no current 
will flow through the galvanometer, b must = c 
Then 

A _ B 
A + R a ~ B+ X a ' 
whence : AB + AX = AB +BR, 

and AX = BR. 

Dividing by BX, we have 

A — K 

B ~ X y 
which is the equation of the ratios between the resistances of the 
arms of the bridge, to insure no flow of current through the gal- 
vanometer. 

The resistance to be measured forms the arm X of the bridge, 
and in order to determine its value the resistances in the various 
arms are adjusted till no current flows through the galvanometer. 
Then the equation just derived holds good and may be solved for X, 

thus X = -^- R. 

The arms A and B are best termed the "ratio arms" of the bridge 
and arm R the rheostat arm. 

In commercial forms of the Wheatstone bridge, A and B are 
usually so arranged that each may be given the values, 10, ioo, and 
iooo ohms, and in some cases I ohm and 10,000 ohms also. 
The ratio arms, A and B, may therefore be adjusted to bear any 

convenient ratio to each other from ■ to , or, in 

1000 10 

. 1 10,000 

some instances, from — to ■ . The rheostat arm 

10,000 1 

is in reality a rheostat capable of being adjusted to any value 
from 1 to about 11,000 ohms. 

In some bridges a sealed battery is furnished with and forms 



a part of the instrument. In those having no battery, suitable 
binding posts are provided, usually marked BB, between which the 
battery may be connected. Other binding posts, usually marked 
XX, are furnished for connecting the terminals of the unknown 
resistance to be measured. 



858 



AMERICAN TELEPHONE PRACTICE. 



Two keys are usually furnished, one in the battery circuit and the 
other in the galvanometer circuit. Each keeps its circuit normally 
open. 

The operation of the bridge is very simple. First some ratio 
between the arms A and B is determined upon. The battery is 
then connected between the proper binding posts, and likewise the 
resistance to be measured is connected between its binding posts. 

The battery key is first depressed and then the galvanometer key. 
A deflection of the galvanometer needle will take place which by 
its direction will after a few trials show whether the resistance in 
the rheostat arm is too great or too small. The rheostat is adjusted 




FIG. 627.— PORTABLE TESTING SET. 



accordingly until the galvanometer needle shows no deflection upon 
the operation of the keys. We then know that our equation 



and consequently 



A_ 
B 



X 



— holds good, 



A 



X R. 



That is, the unknown resistance is equal to the ratio between B 
and A multiplied by the resistance in the adjustable arm. 

Considerable judgment may be exercised in the choosing of 
the appropriate ratio in the ratio arm to obtain the greatest accuracy. 
Obviously, if a high resistance is to be measured the ratio should 
be large, and vice versa. 



TESTING. 859 

In bridges having resistances of 10, ioo, and iooo ohms in the 
ratio arms, the following values in arms A and B will give the 
best results : 

Resistance to be measured. 

Under ioo ohms, 

ioo to iooo ohms, 

iooo to 10,000 ohms, . 

10,000 to 100,000 ohms, 

100,000 to 1,000,000 ohms, 

As to the accuracy of measurements attainable by the use of 
the Wheatstone bridge, the following table represents the claim 
of one reliable manufacturer: 

.01 of an ohm to an accuracy of 1 per cent. 

u a u a a jl " ct 

" l " " 

a a (i 1 a a 

5 

a a a -^ a a 





A arm. 


B arm. 


. 


IOOO 


10 


. 


IOOO 


IOO 


. 


IOOO 


IOOO 




IOO 


IOOO 




10 


IOOO 



I 


ohm 


IO 


ohms 


IOO 


11 


IOOO 


a 


10,000 


u 


100,000 


a 


1 ,000,000 


a 



a a << \ a a 

4 

, s „ „ 

If using the no volt lighting circuit as battery power 1 meg- 
ohm may be measured accurate to \ per cent. 

There is no doubt that with a well-made bridge with a sen- 
sitive galvanometer, these results may be equaled if not surpassed. 
Great care must be taken in using a voltage as high as no, as 
there is danger of burning out the coils. Such high voltage should 
be used only in measuring very high resistances, and the ratio arms 
should be adjusted to give as high a multiplying ratio as pos- 
sible. 

A particular form of bridge which has come into extensive use in 
this country and which possesses several unique features is shown 
complete in Fig. 627 and in plan view in Fig. 628. 

The various adjustments of the arms are accomplished by placing 
plugs in the various holes between the brass blocks arranged in rows 
as shown in the latter figure. Between each successive pair of blocks 
are arranged resistance coils having the resistance in ohms desig- 
nated on the plan. Placing a plug in a hole between two blocks short- 
circuits the resistance connected between those two blocks. The 
rheostat arm of this bridge is represented by the top and bottom 
row of blocks, and if all plugs are removed the resistance in this arm 



860 



AMERICAN TELEPHONE PRACTICE. 



will amount to 11,110 ohms. The ratio arms A and B are repre- 
sented by the left and right-hand halves respectively of the center 
row. A galvanometer and suitable battery, together with battery 
and galvanometer keys, are all mounted in a carrying case as shown. 
The connections of this instrument are indicated in Fig. 628, and 
are as follows : The top row of blocks is connected to the bottom 
row by a heavy copper bar joining the right-hand blocks. This 
connection is made very heavy so as to interpose no extra resistance 
in the rheostat. On the rheostat formed by the upper and lower 
rows of blocks any resistance from 1 to 11,110 ohms may be obtained, 
the resistance being added by leaving out plugs. The lower left- 
hand block of the rheostat is connected to the lower binding post, D, 
forming one terminal of the unknown resistance. The upper post, 



7B 



QDODDOO££T ■] 







FIG. 62S.-PLAN OF PORTABLE TESTING SET. 



C, forming the other terminal of the unknown resistance, is con- 
nected to block, X, which block is also joined to the galvanometer 
key. The block, R, is connected to the upper left-hand block of 
the rheostat. The end blocks of the middle row are connected to- 
gether and to the + terminal of the battery. The — terminal of 
the battery is connected through the battery key to the lower left- 
hand end of the rheostat. One galvanometer terminal is connected 
directly to the block, R, the left-hand block of the rheostat, and to 
the back contact of the galvanometer key. The other galvanometer 
terminal is connected through the key to the block, X. 

By carefully following out these connections it will be apparent that 
the parts as connected form three arms of a Wheatstone bridge, the 
fourth, of course, being the unknown resistance joined to the line 



TESTING. 



861 



posts. This is shown diagrammatically in Fig. 629, where the cor- 
responding parts are similarly lettered. 

It will be noticed that this latter figure is practically the same 
as Fig. 626, with the addition of the center blocks, A, B, X, R, 
forming a sort of commutator. The object of this arrangement 
is to make it possible to reverse the connections of arms, A and 
B, with R and X. Thus with the plugs in the position shown by 
the black dots the connection is precisely as shown in Fig. 626, 

and the equation — = — holds true. If, however, the plugs are 

inserted in the holes on the other diagonal, arm, A, will be con- 
nected to arm, X, and arm, B, to arm, R, and the equation of 

/? /? 

the bridge will be — - == -^. 
fe AX 

The bridge arms, A and B, have not the same range of resist- 
ances in this bridge, A, having only 1, 10, and 100 ohm coils, 




FIG. 629.— CIRCUITS OF PORTABLE TESTING SET. 



while the resistances of B are 10, 100, and 1000 ohms. Therefore, 
if a ratio of 1000 to 1 for measuring large resistances is desired, 
the plugs are inserted in the commutator along the arrow H (Fig. 
628) ; while an opposite arrangement of the plugs along the arrow 
L will give a ratio of 1 to 1000 for very small resistances. In this 
bridge the galvanometer key is so arranged as to short-circuit the 
galvanometer while the key is up. 

The galvanometers usually furnished with the complete bridges 
consist of a needle so mounted as to swing freely in a hori- 
zontal plane. This needle is given a tendency to point in one 
direction, sometimes by the action of the earth's magnetic field and 
sometimes by the field of a powerful permanent magnet. By caus- 
ing the current through the bridge wire to flow through a coil. 



862 AMERICAN TELEPHONE PRACTICE. 

either stationary and surrounding the needle, or movable and car- 
ried on the needle, the needle is caused to swerve from its normal 
position and to place itself at right-angles to the lines of force due 
to the permanent field. The deflection of the needle is great or small 
according to the strength of the current, and to the right or left 
according to the direction of the current. 

In many of the tests to be described later a galvanometer of 
greater sensitiveness is required, and some form of reflecting in- 
strument is used. In these the needle carries a small circular 
mirror, which reflects a spot of light from a lamp or some other 
source against a scale. In this arrangement every movement of 
the needle causes the spot of light to move along the scale, and 
a little consideration will show that the angle through which the 
reflected ray of light moves is double that through which the needle 
travels. Thus this reflected ray of light serves as a needle of any 
desired length, and has the advantages of magnifying the angular 
movement of the needle to twice its real amount, and of possessing 
no mass, and therefore no inertia. 

The two galvanometers used to the greatest extent for quanti- 
tative measurements in practical work are the Thomson and the 
D'Arsonval. 

The Thomson galvanometer is made in a great variety of forms. 
The needle consists of several very light bar-magnets arranged side 
by side and with opposing poles together, so that the directive influ- 
ence of the earth's field shall be very slight. The needle is directly 
attached to a small silvered glass mirror, and is suspended within 
the coil or coils by means of a silk or quartz fibre. The current to 
be measured is passed through the coils, and the magnetic field set 
up thereby causes the needle to swerve from its normal position. 
The Thomson galvanometer is used in the most delicate tests, and 
is essentially a laboratory instrument. It has the disadvantage 
of being affected to such an extent by external magnetic fields as to 
render its use impossible in many cases. A passing street car or 
variations in the current flowing in a neighboring circuit will cause 
the needle to swing violently, thus making accurate work out of the 
question. These disadvantages may be overcome to some extent 
by inclosing the galvanometer in a heavy iron case — such as an old 
safe — but they tend to make it a very undesirable instrument for 
portable work. Where the instrument can be permanently set up 
and properly guarded, it is unequaled for delicacy and accuracy. 

For nearly all practical engineering work, the D'Arsonval gal- 



TESTING. 



863 



vanometer is sensitive enough, and has the advantage of being much 
more convenient for general work. In this the needle is a coil 
instead of a permanent magnet, and is suspended within the field 
of a powerful permanent magnet instead of in a coil. The needle 
carries a mirror, as in the Thomson instrument. The current to 
be measured is passed through the coil, and as this coil lies in the 




FIG. 630.-D'ARSONVAL GALVANOMETER. 



field of the permanent magnet, a rotation of the coil ensues, the action 
being identical with that which causes the armature of an electric 
motor to revolve. 

In Fig. 630 is shown a much-used form of D'Arsonval galvano- 
meter made by Queen & Co., Philadelphia. The field is built up of 
a number of horizontal permanent magnets, between the polos of 
which is suspended the needle. The needle system is shown in 



864 



AMERICAN TELEPHONE PRACTICE. 



detail in Fig. 631. It consists of a coil of wire, W, wound on a box- 
wood frame, D, and supported by means of the flat phosphor-bronze 
filament, A, from the torsion pin, E. The current is led in by 
means of the torsion pin, E, and suspension wire to the coil ; thence 
to the spiral spring, B, and by means of the bottom contact out to 
the external circuit. A ring, F, is joined above the coil frame, and 
another, G, below the coil frame. These are normally a sufficient 




FIG. 631.— SUSPENSION OF D'ARSONVAL GALVANOMETER. 



distance apart to enable the system to swing freely, but when pack- 
ing for transportation the torsion head may be pressed down until 
the rings above mentioned firmly clamp the coil. In this condition 
it will withstand shipment satisfactorily. To the right is shown the 
coil more clearly. The two points, U and L, have soldered to them 
the ends of the coil, IV. The mirror is shown at C. 

The great advantage of the D'Arsonval galvanometer is that 
it is unaffected by variations in the external magnetic field. It may 



TESTING. 



865 



even be used close to dynamo machinery without being sensibly 
affected. 

In order to read the deflection produced by a current, in any form 
of reflecting galvanometers, two method's may be employed. One 
is to cause the needle to reflect a spot of light from a stationary 
source, upon a horizontal scale, and by watching the movement of the 
spot the number of scale divisions deflection may be accurately de- 
termined. Another and better way is to focus a telescope on the 
mirror, in such manner that the horizontal scale will be visible in the 
telescope. The mirror in its movements will reflect different por- 
tions of the scale into the telescope, and the deflection may thus be 




FIG. 632.— SCALE AND TELESCOPE. 



observed with great precision. When this method is used the num- 
bers on the scale should be reversed, in order to appear normal in the 
telescope. Fig. 632 shows a telescope and scale as arranged for this 
purpose. 

Complete testing sets, containing reflecting galvanometers, 
bridges, batteries, keys and other accessories, are frequently mounted 
in one case, and so arranged as to fold within small compass when 
not in use. This arrangement is very convenient, but has one dis- 
advantage — the manipulation of the keys and plugs jar the box to 
such an extent as to make the readings on the galvanometer unrelia- 
ble. The separately mounted galvanometer is therefore in general 



866 AMERICAN TELEPHONE PRACTICE. 

to be preferred. Of course, this applies only to reflecting galvano- 
meters. 

It is frequently found that a current that it is desired to measure 
is so large that it sends the spot of light completely off the scale, 
thus rendering the measurement of the deflection impossible. In 
order to increase the range of the galvanometer so as to make it 
available for measuring both large and small currents, certain resist- 
ances called shunts may be placed in parallel with the galvanometer 




FIG. 633.— GALVANOMETER AND SHUNT. 

coil as in Fig. 633. The resistance of the shunt being, known, it is 
easy to calculate the amounts of the currents that pass through the 
galvanometer coil and the shunt. 

Calling R g the resistance of the galvanometer, R s that of the 
shunt, I g the current through the galvanometer, I s that through 
the shunt, and / the total current through both, then 

/= /„ + /„ 

Also when E is the difference of potential between the common 
terminals of the galvanometer and shunt, 

I t = -§- and /. = A. 

I^R Z 



E= LR g = I s R % . Hence I s = 

Substituting this value of I S} in the first equation we have 
/_ j 4- ^g^g _ / ( T j. ^s \ _ / ^ + R g 



= 7 -( I + f) = / ' 



§ r s —~*y RJ~ s R, 

R + R 
The quantity s — - is called the multiplying power of the 
R s 

shunt because it represents the number by which the current through 

the galvanometer must be multiplied, in order to give the value of 

the current being measured. 

Shunt boxes are usually provided for a given galvanometer with 

a number of coils specially arranged to give such convenient values 

of the multiplying powers, as 10, 100, and 1000. For this purpose 



TESTING. 867 

the various coils of the shunt box have resistances of £, *V, and m 
of the resistance of the galvanometer. 

To better show this relation, assume that a multiplying power 
of iooo is desired, then 

i? s + R 



iooo 


— 




Rs 


• 


oooR s 


— 


R s 


= 


x s 




R, 






A 



iooo - i 999 

A commercial form of shunt box is shown in Fig. 634, the various 
multiplying values of the shunt being obtained by plugging the block 
corresponding to the multiplying power desired. 

For moderate deflections, the current traversing the coils of a 
reflecting galvanometer may, without sensible error, be taken as 
proportional to the deflection of the spot of light on the scale, or 




FIG. 634.— SHUNT BOX. 

to the deflection read through the telescope. The current is, of 
course, inversely proportional to the total resistance of the circuit, 
and from this it follows that the deflections are inversely proportional 
to the resistance. This fact enables the galvanometer to be used for 
measuring unknown resistances by comparing the deflection obtained 
when a given E. M. F. acts through a known resistance with that 
obtained when the same E. M. F. acts through an unknown resist- 
ance. 

The general method of measuring resistances by the use of a 
galvanometer is to note the deflection obtained with a given bat- 
tery and a known resistance in the circuit, and from this to com- 
pute what is called the working constant. This working constant 
may be defined as the number of scale divisions deflection that would 
be obtained by causing the current from the given battery to pass 



868 AMERICAN TELEPHONE PRACTICE. 

through the galvanometer and a resistance of one megohm. Of 
course such a deflection as this can exist in our imagination only, 
but it serves, nevertheless, as a convenient standard upon which 
to base our calculations. Having obtained the working constant, 
a reading is taken of the deflection produced by passing the battery 
current through the galvanometer in series with the unknown resist- 
ance. As the deflections are inversely proportional to the resist- 
ances, the unknown resistance is then readily computed. 

If measurements of comparatively low resistance are to be made, 
then the resistance of the battery and of the galvanometer must be 
taken into consideration as well as that of the resistance placed in 
circuit with them, but as the measurements here considered will be 
those of very high resistances only, the resistance of the battery and 
of the galvanometer may be neglected. For the purpose of taking 




FIG. 635.— CIRCUITS FOR GALVANOMETER CONSTANT. 

the constant, connections are made as shown in Fig. 635, where B is 
the battery, G the galvanometer, 5 the shunt, and R the known 
resistance. Usually the value of R is to of a megohm, or 100,000 
ohms. With the vfa shunt a certain deflection will be obtained 
when the circuit is closed. Obviously, if the shunt were not present 
the deflection would be 1000 times as great, because only -nnnr of 
the current passes through the galvanometer. Therefore the total 
deflection, if it could be measured, that would be produced through 
the galvanometer and the 100,000 ohms resistance, would be the 
deflection noted multiplied by 1000. If, now, the resistance, R, 
had a value of 1 megohm instead of tV megohm, the deflection would 
have been only xV as great as this. Therefore to find the number 
of scale divisions deflections which the galvanometer alone would 
give with 1 megohm in circuit, we multiply the deflection noted by 
1000 and by to. 

In general we may say: to find the working constant, multiply 



TESTING. 869 

the deflection obtained by the multiplying power of the shunt, and 
by the value of the known resistance in megohms. 

As a numerical example let us assume that with the m? shunt 
and the iV megohm resistance, we obtain a deflection of 2000 scale 
divisions, then the working constant is 

1 

200 X lOOo X — = 20,000. 

10 

In other words, 20,000 would be the number of scale divisions 
obtained were the entire current from the battery allowed to pass 
through the galvanometer with one megohm in series. 
. With 50 cells of battery (45 or 50 volts), the constant under 
ordinary working conditions with a good D'Arsonval galvanometer, 
will be from 10,000 to 25,000. With a Thomson instrument a much 
higher constant may be obtained. Mr. George D. Hale of the 
Western Electric Company's cable-testing department, uses a large 
four coil Thomson instrument with 600 volts obtained from a motor 
generator. With this he obtains a constant of 528,000, and by 
adjusting the suspension for greater delicacy can obtain as high as 
2,000,000. Of course this is entirely impracticable for portable 
instruments, and is, in fact, unnecessary, as good work may be done 
with a constant of 20,000. In ordinary testing a battery of 50 cells 
is sufficient. Of course a higher working constant may be obtained 
with a larger battery, and frequently 100 cells are used. 

INSULATION TESTS. 

One of the principal uses of the galvanometer in line testing is in 
the measurement of insulation resistance. The insulation resistance 
of any line or conductor is the joint resistance of all the leaks from 
the line to the ground or to other conductors. On a pole line every 
insulator forms a leak to earth, and on a line having 40 poles to the 
mile there would be 40 such leaks in parallel. The insulation resist- 
ance of a line as a whole varies inversely as its length, if the insula- 
tion is uniform. Evidently, a line two miles long would have one- 
half as great an insulation resistance as a similar line one mile long, 
because on the latter there would be only half as many leaks as on 
the former. In general it may be stated that a line n miles long 

will have only — as great an insulation resistance as a similar line 

one mile in length. In order to obtain a standard of insulation 
resistance independent of the length of the line, it is convenient 
to express the insulation resistance as so many megohms per mile. 



870 



AMERICAX TELEPHONE PRACTICE. 



The insulation resistance per mile is found by multiplying the insu- 
lation of the line as a whole by the length of the line in miles. 

In order to measure the insulation resistance of a line the con- 
stant of the galvanometer is first taken and then the known resistance 
is cut out of circuit and the line insulation resistance substituted for. 
it. Assuming that the insulation resistance to be measured is that 
of a wire in a cable, the terminals of the circuit which were connected 
with resistance, R, in Fig. 635, will be connected one with the wire 
and the other with the sheath of the cable as shown in Fig. 636. 
Care must be taken that the wire being measured is carefully insu- 
lated from the sheath at the other end of the cable. The shunt, S, 
is then cut out of circuit in order that the full current may pass 
through the galvanometer. Before completing the circuit with the 
cable conductor and sheath, however, the key, K, should be closed 



TAPE 
lN5UL*TIO^ / LEAD SHEATH, 




ca.bl.el 



FIG. 636.— INSULATION RESISTANCE OF CABLE. 



in parallel with the galvanometer, in order to prevent the rush of 
current that will take place in charging the cable, from causing 
the needle to give too violent a kick. After a short time the key is 
opened and all of the current diverted through the galvanometer. 
The galvanometer then receives only that current which leaks from 
the core of the cable to the sheath through the insulation. Under 
these circumstances a certain deflection will be noted, and by compar- 
ing this deflection with the constant already obtained the value of the 
insulation resistance in megohms is readily determined. 

To illustrate, suppose that a deflection of 75 scale divisions is 
obtained with the apparatus connected as in Fig. 636. If the con- 
stant is 20,000, as already determined, we know that the insulation 
resistance must be 20,000 divided by 75, or 266 megohms, thus 
indicating that the total insulation resistance of the cable is 266 meg- 
ohms. That this is true is evident from the fact that the constant, 



TESTING. 



871 



20,000, represents the number of scale divisions deflection that would 
be obtained were only one megohm in the circuit. The deflections 
are inversely proportional to the resistance in the circuit, and there- 
fore the total insulation resistance is equal to the deflection 
through one megohm divided by the deflection through the insula- 
tion resistance, or 20,000 divided by 75. To sum up these opera- 
tions : 

1st. Obtain the galvanometer constant or deflection obtained when 
the galvanometer in series with one megohm resistance is subjected 
to the potential of the battery. 2d. Find the deflection obtained 
when the galvanometer and insulation resistance in series are sub- 
jected to the potential of the battery. 3d. Divide the constant by the 



LEAD 1 SHEATH 




BZ= 



FIG. 637.— CONNECTION FOR INSULATION TEST. 



deflection obtained through the insulation resistance, the result being 
the insulation resistance of the cable expressed in megohms. 4th. 
To find the insulation resistance per mile, multiply the total insula- 
tion resistance by the length of the cable in miles. 

If the insulation of the cable is low, a shunt must be used in 
obtaining the deflection through the insulation resistance. If the 
insulation resistance is high, the deflection will be small and no 
shunt will be required. The purpose of the shunt is merely to keep 
the deflections on the scale so that they may be read. 

In Fig. 637 is shown a convenient arrangement of connections 
for making insulation tests. In this, B is the battery of say 50 
cells, R the T \ megohm box, 5 the shunt box, G the galvanometer. 



£72 AMERICAN TELEPHONE PRACTICE. 

and V a convenient switch for throwing either the fa megohm box 
or the line insulation into circuit with the galvanometer and bat- 
tery. When the levers of the switch, V , are in the position rep- 
resented by the dotted line, the circuits are those for taking con- 
stant of the galvanometer, and when in the position shown by full, 
lines, the circuits are those for obtaining the deflection through the 
insulation of the cable. Various forms of keys for changing the 
direction of the battery current through the galvanometer, and for 
performing other switching operations with the greatest possible 
convenience, are obtainable, and form an important part of all test- 
ing outfits. The scope of this work will not permit of their detailed 
description. 

In making insulation tests the resistance of the lead wires to the 
cable or line need not be taken into account. It is a matter of the 
greatest importance, however, that these wires be perfectly insu- 
lated from each other. It is a very easy matter in making tests of 
this nature to measure the wrong quantity. 

One very important matter in connection with the insulation tests 
has not yet been spoken of. When the reading is being taken, 
with the cable or line insulation in circuit, it will be noticed that 
a maximum deflection is obtained at first, and that this gradu- 
ally diminishes, as though the insulation resistance were increas- 
ing. This is due to what is called electrification, a phenomenon 
that is not very thoroughly understood. When the electromo- 
tive force of the battery is first applied to the cable or line, 
there is a sudden rush of current, due to the charging of the con- 
ductors. The charges, however, apparently soak in to the insu- 
lation to a slight extent, thus allowing more current to flow to the 
conductors. After the first rush due to the first charging of the 
conductors, there is still a flow of current, due in part to this soak- 
ing in, and in part to the actual leakage through the insulation. 
It is the current due to the latter that we are concerned with in insu- 
lation measurements, and therefore we must wait till the soaking 
in process ceases, when the flow of current will be practically con- 
stant, being that through the insulation. In nearly all telephone- 
testing work, one minute is allowed for electrification, after which 
the reading is taken of the deflection. When one is thoroughly 
familiar with his instruments he may often, where great accuracy 
is not required, estimate what the deflection at the end of one minute 
will be, by watching the deflection for 30 or 40 seconds. This 
method saves time, but must be used with extreme caution. 



TESTING. 873 

With a constant of 20,000 a reading taken on a wire in a piece 
of good new telephone cable, one-quarter mile long, would probably 
show a deflection of 8 or 10 scale divisions upon the closure of the 
key. This would decrease to about 6 scale divisions in 2 seconds, 
and to about 2 scale divisions in 30 seconds, after which it would 
remain constant. The reading of 2 divisions at the end of the 

ii-i- ,. . . . 20,000 

minute would indicate an installation resistance 01 = 10, cod 

2 

megohms, or 2500 megohms per mile. 

As examples of deflections on the different wires in various cables 
the following are given : 

Dry cable, J mile long, two years old. Galvanometer constant 
22,000: Readings, 12 — 15 — 15 — 10 — 10 — 15 — 15 — 15 — 13 scale 
divisions. 

Another dry paper cable, 2750 feet long one year old. Gal- 
vanometer constant 19,000: Readings, 5 — 6 — 5 — 4 — 5 — 5 — 5 — 5 — 
6, etc. 

A piece of jute and ozite cable five years old, 6000 feet long, gave 
the following with a constant of 20,000: 7500 — 2500 — 1000 — 1000 
— 600 — 800 — 900 — 800 — 1000. It was necessary to use the tV shunt 
in taking these readings. 

Another piece of the same kind of cable, 800 feet long, with a con- 
stant of 20,000, gave 175 — 200 — 250 — 270 — 160 — no — 120 — 160 — 
no — 125. 

CAPACITY TESTS 

A very important measurement, especially in telephone cables, 
is the determination of the capacity of the line conductors with 
respect to all neighboring conductors. The usual method of making 
capacity tests is to note the deflection produced when a condenser 
of known capacity, after having been charged to a known potential, 
is discharged suddenly through the galvanometer, and to compare 
this with the deflection obtained when the cable or conductor being 
measured, after being charged to the same potential, is discharged 
through the galvanometer. The deflections produced under these 
circumstances are proportional to the charges, and therefore to the 
capacities of the standard condenser and the line or cable. The 
circuits for obtaining the deflection produced by the discharge of the 
condenser are shown in F.g. 638, where C is the standard condenser. 
B the battery, and G the galvanometer. When the key is depressed 
the condenser is charged to the full potential of the battery, B. The 
key is then suddenly released, thus allowing the charge from the 



874 



AMERICAN TELEPHONE PRACTICE. 



condenser to pass through the galvanometer, thus producing a cer- 
tain throw of the needle. The connections are then made as shown 
in Fig. 639, the same battery, B, being used. When the key is 
depressed the cable is charged, and when suddenly released this 
charge flows through the galvanometer and produces another throw 
of the needle. By comparing the throw produced by the charge of 




|l|i|i|l|l 

B 
FIG. 638.— CAPACITY TEST. 



the condenser with that produced by the charge of the cable, a direct 
comparison may be made between the capacity of the cable and that 
of the condenser. Thus, if with the 979 shunt the discharge from 
the condenser gave a deflection of 100 scale divisions, the capacity 
of the condenser being vs microfarad, and if with the same shunt 



LEAD SHEATH- 




FIG. 639.— CAPACITY TEST FOR CABLE. 



the discharge of the cable produced a deflection twice as great, we 

would know that the capacity of the cable was 2 X — ~ - micro- 
farad. IO 5 

Convenient connections for making capacity tests are shown in 
Fig. 640, where G is the galvanometer, 5 the shunt, C the stand- 
ard condenser KK discharge keys, V the selecting switch and B, 



TESTING. 



875 



a battery of eight or ten cells. With the switch, V , at the left and 
both discharge keys depressed, the current from the battery will 
flow into the condenser, thus charging it. Upon the sudden release 
of the discharge keys, the condenser will discharge through the gal- 
vanometer and shunt, giving a deflection which should be noted. With 
the switch, V , at the right, the cable may be charged or discharged in 
the same manner, and the deflection produced by its discharge noted. 
About seven cells of battery is usually sufficient for making capacity 
tests on telephone cables. If a non-adjustable condenser only is 
available, one having a capacity of to microfarad is probably most 
desirable. For accurate work a subdivided condenser, having its 



»NSULAT.O N 7 A r E LEAD SHEA 



<-S_lSZ3-\ 





CABLE. 



L#6fr-J 



FIG. 640.-CIRCUITS FOR CAPACITY TEST. 



divisions so arranged as to be easily connected in multiple or in 
series, or in combinations of the two, is very desirable. Then the 
condenser capacity may be varied until the throw from the con- 
denser is nearly equal to that from the cable, thus greatly minimiz- 
ing the liability to error in the results. In making capacity tests the 
wire under test should be carefully insulated and all the other wires 
in the cable should be connected together and to the sheath or ground. 
Fifteen seconds should always be allowed for the charging of the 
cable. 

If d is the throw due to the discharge of the condenser, d' that 
to the discharge of the cable, K the capacity of the condenser in 



876 AMERICAN TELEPHONE PRACTICE. 

microfarads, and X the capacity of the wire being measured, then 

X : K :: d' : d 

X=^K. 
a 

If the throws of the galvanometer are too large to be measured, 
the shunt must be used. In this case d or d' in the formula will 
be the actual throws observed multiplied by the multiplying power 
of the shunt. 

THE LOCATION OF VAULTS. 

When a break occurs in a wire in a line or cable, the ends remain- 
ing insulated from other wires and the ground, the only recourse is 
to capacity tests. The capacity of the two parts of the wire will be 
proportional to their lengths, the wire being uniform in size and in 
its relation to other wires, throughout its length. 

We may locate a break of this nature in several ways. 

Measure the capacity of one end of the broken wire, then go to 
the other end of the cable and do the same. Calling D the length 
of the cable in feet, C the capacity of the first portion of the wire, C 
that of the other, and X the distance in feet to the break from the 
first end, then : 

X : D :: C : C + C 

A V CD 

and X = c+ c , . 

When a good wire is available, and this is usually the case, set 
up the instruments for capacity for testing, and take a throw, d, 
on the broken wire, another, d', on the good wire, and a third, d" , 
on the good wire with the broken wire connected to it at the far end. 

Evidently the throw on the whole broken wire would be d" — d' 

+ d. 

Hence where D and X have the same significance as before 
X : d : : d : d" — d' + d 

andX dD 



d" — d + d 

The location of breaks is much complicated by the presence of 
poor insulation between ruptured portions, and between other wires. 
The insulation resistances between these parts should always be 
taken. If less than one megohm, the results obtained by the capacity 
tests should not be relied on, and other methods too complex for 
description here may be resorted to. It seldom pays to open a lead- 
covered telephone cable for the purpose of joining a few broken wires, 



TESTING. 



877 



the expense of making the splice being usually in excess of the value 
of the wires. 

The location of crosses or grounds is rendered somewhat difficult 
by the fact that there is nearly always some resistance in the fault 
itself. If we know the resistance of the defective wire and have no 
good wire running parallel with it, we may proceed as follows, using 
a good Wheatstone bridge : 

Measure the resistance of one end of the defective wire through 
the fault to ground. Do the same at the other end. Then calling R 
the total resistance of the wire (either known or calculated from its 
size and length), R' the measured resistance from the first end, R" 
that from the other end, X the resistance from the first end to the 




BAP WIRE x 



<5QOP WIRE. C 






FIG. 641.— VARLEY LOOP TEST. 



fault, Y the resistance from the second end to the fault, and Z the 
resistance of the fault, we have : 

R = X + Y. 
R> = x + Z. 
R" — Y + Z. 
Solving these for X and Y we have 

R + R' — R" 



X = 



Y= 



R— R' + R" 



which values are independent of the resistance of the fault. Know- 
ing the resistance to the fault, it is easy to compute the distance to 
it, from the resistance per foot of the conductor. 

When a good wire is available, the Varley loop test should be 
used, as it is more accurate than the method just described. For 
this a Wheatstone bridge is used, and connected as in Fig. 641. The 
good and bad wires are joined at their distant ends, and one terminal 



878 AMERICAN TELEPHONE PRACTICE. 

of the battery connected to the point, c, on the bridge, while the other 
terminal is grounded. It is not difficult to see that the partial ground 
or fault now bears the same relation to the bridge as the point, i, in 
the diagram of Fig. 626 ; the rheostat arm now includes the resist- 
ance R, plus the resistance of the bad wire to the fault, while the 
unknown arm includes the resistance of the good wire, plus the resist- 
ance of the bad wire on the other side of the fault. 

The equation of the bridge, when balanced, then becomes 
A _ R + X 
B ~ C+ Y' 
where R is the unplugged resistance of the rheostat, X the resist- 
ance to the fault, Y the resistance beyond the fault, and C that of the 
good wire. 

Now calling L the resistance of the loop consisting of the good and 
bad wires, we have 

L = X+Y + C, 
or C + Y = L —X. 

Substituting this in the second member of the equation of the 
bridge, we have 

A R + X 



whence X = 



B L — X' 
A L— B R 



A + B ' 

which is independent of the resistance of the fault. When the two 
ratio arms of the bridge are given equal values we have A = B, and 
the equation for X becomes : 

2 

L may be known from records previously made, may be computed 
from the size and length of the wires, or, if only one ground is 
present on the bad wire, it may be measured directly on the bridge. 

Sometimes, in ordinary paper cables, a requirement is made that a 
rubber-covered test wire shall be run through the center of the cable, 
so that at least one good wire may always be available in testing. 
Where no good wire is available, a separate wire may be strung to 
be used as the return in this test. 

If the lead wires, from the instruments to the faults' wire, have 
appreciable resistance, this should be measured, and deducted from 
the value of X. After this the distance to the fault may be readily 
obtained from the resistance per foot of the conductor. 



TESTING. 



879 



A second Varley method, quite as good as this, and very valuable 
as a check method, is the following: The apparatus is arranged as 
in Fig. 642, and it will be noted that the conditions are similar to 
those of the first method, and that only a reversal, at the set, of the 
good and bad wires is required. 

With this arrangement, the bridge equation, when a balance is 
reached, must be 

the letters referring to the same things as before. 

again calling L the loop resistance of the good and bad wires, we 

have 

L = X+Y+C, 
ovC+Y — L — X. 




SAD WlfTE X 



/ V 



qoop vyi^E Q 



FAUUT'i" 



FIG. 642.— VARLEY LOOP TEST— CHECK METHOD. 



Substituting this in the second member of the last form of the equa- 
tion, we have 

A (R+ L—X)—BX, 

whence X= A + B 

This result also is independent of the resistance of the fault, and 
it may be said of both Varley methods that the fault resistance may 
be one which changes its value from moment to moment, without 
changing the final result. Also the potentials of the two ground 
connections with reference to the bad wire may change from mo- 
ment to moment and not harm the result. Indeed, it is possible 
sometimes to locate a fault with no battery at all, as the difference 
of potential between the two grounds is frequently great, where 
grounded trolley systems exist. 

These two methods ordinarily will be applied to pairs of wires 



880 



AMERICAN TELEPHONE PRACTICE. 



forming a loop of which the two sides are about alike ; this is the case 
in testing cable conductors for a ground on one or more, when one 
good cable wire exists ; it is also true in testing for a ground on one 
wire of a metallic circuit open wire line, the other wire being good. 
In these cases the resistance to the fault always will be less than 
the whole resistance of either wire. In the second Varley form, 
therefore, the bridge arms can never be equal, with a ratio of i, 
but the arm A must always be greater that the arm B. 

VARLEY TEST BY ARITHMETIC. 

The great certainty of the Varley tests in locating grounds and 
crosses make them of value to all persons having, to do with telephone 
lines ; by using the following simple rule, anyone who can add and 
divide by two can make fault locations by the first method. The con- 
nections are shown in Fig. 641 and the conditions are that the good 
and bad wires shall be alike and the bridge arms equal : 

Rule. — Adjust the rheostat till the deflection is the smallest possi- 




BAD WIRE X 



GOODWinE 



>~ 



FAULT "*& 



FIG 643.— MURRAY LOOP TEST. 



ble; read the resistance in the rheostat and divide it by two; the result 
is the resistance from the fault to the further end of the wire under 
test. 

It must be observed that the result is not given in ohms from the 
testing point to the fault, but in ohms from the fault to the other end. 
This is as serviceable, however, and is sometimes just what is wanted. 

There is another loop test not quite so convenient as the Varley 
for those possessing only an ordinary bridge, but available and relia- 
ble where one has a standard resistance box and a galvanometer. 
This is known as the Murray loop test and connections for it should 
be made as shown in Fig. 643. The point e may be a plug inserted 
into an intermediate hole in a standard resistance box, the points 
/ and h being the end terminals of the box. If two separate resist- 



TESTING. 881 

ances are available, the two may be connected in series, the point 
between them being then represented by e, in Fig. 642. 

The two resistances are manipulated until a balance is obtained, 
no current being indicated by the galvanometer, when 

A X 

B ~ C + K' 
the significance of the various letters being the same as used in 
describing the Varley test above. From this is obtained 

A L 



whence X 



B ~ L — X y 
A L 



A + B y 

in which L is as before the resistance of the loop. This value of X 
is independent of the resistance of the fault. 



INDEX. 



A 



board, express system, 252 
operator defined, 355 



Ader type of receiver, 39 
Aerial cable construction, 805 
Ahearn transmitter, 71 

American Electric, common battery multi- 
ple, 346 
Ampere, discovery of electromagnetism, 1 
Arago, progress of knowledge in electro- 
magnetism, 1 
Automatic complete circuit diagram, 720 

• Electric Company's equipment, 694 

■ ringing circuit, Kellogg, 372 

■ ringing circuit, Western Electric, 

365 
•, schematic circuits, 711 

sub-station equipment, 702 

systems, 691 * 

vs. Manual, 727 

B -board, express system, 254 
Baird coin collectors, 462 
Battery, dry, 96 

Fuller, 90 

, gravity, 94 

, LeClanche, 86 

Batteries, primary, 85 

, primary, method of test, 99 

Bell, Professor Alexander Graham, inven- 
tion of telephone, 7 
Berliner, Emile, transmitter, 14 

Universal transmitter, 59 

Biased ringer, 129 

Blake, Francis, transmitter, 55 

Bourseul, Charles, prediction of electric 
telephony, 5 

Bridging local battery sub-station equip- 
ment, 137 

Butt plates, 770 

Busy-back attachment, 564 

B-W-C party line, 441 



c 



able capacity, table, 810 

fanning, 655 

, rubber covered, table, 8' 

splicing, 820 

— terminals, 830 

testing, 851 

• weights, table, 812 

Calling apparatus, magneto, 104 



Capacity, 23 

, table of specific inductive capac- 
ities, 29 

tests, 873 

Carbon block arresters, 590 

, theories of transmitter action, 53 

Carty bridging bell circuit, 426 

, J. J., on disturbances in grounded 

lines, 158 
Central office lay-out plaza diagram, 674 
Charging machines, 545 

■ • storage batteries, 582 

Chief operator's equipment, 628 

Clamond transmitter, 58 

Cleveland divided multiple, circuit, 384 

switch-board, general view, 393 

Clock circuit for battery test, 99 

Coil, induction, circuit for comparative 
tests, 83 

, induction, for local battery tele- 
phones, 73 

, induction, table of data, 79 

, induction, table of various makes, 

81 

, induction, Varley wound, 77 

, induction, with transmitter, 21 

, ringer, Varley, 115 

Coin collecting devices, 462 

Colvin transmitter, 67 

Combined drop and jack, Kellogg, 203 

drop and jack, Western, 197 

drop and ringer, 206 

Common battery divided multiple system, 

390 
battery feed for local storage cell, 

276 

■ battery line signals, 279 

battery multiple switch-board, 313 

battery signaling, 277 

battery sub-station equipment, 304 

battery, sub-station, Western Elec- 

tric circuit, 307 

■ battery supervisory signaling, 2S4 

battery switch-boards for small ox- 
changes, 293 

battery transmission systems, 265 

Condensers, construction of, 30 
Condenser, value in telephonic design. 33 
Conduit construction, S35 
— costs, table, S49 



hS4 



INDEX. 



Connelly & McTighe, G91 
Controller coin collector, 471 
Cooper Hewitt telephone repeater, 750 
Cord atachment, 522 

connectors, 524 

tips, 50 

weight, 525 

Cortlandt street, New York, circuits, 231 

switch-board, detailed view, 352 

Court decision on Reis's rights, 11 
Credit meter, 479 

Cross arms, table of sizes, 773 
Crossley transmitter, 57 
Currier and Rice, party line, 449 

D'Arsonval galvanometer, 863 
Davy, progress of knowledge in elec- 
tromagnetism, 1 
Dean common battery switch-board circuits, 
298 

common battery transmission sys- 
tem, 273 

Decision Supreme Court on Reis's rights, 

11 
Desk set wiring, 148 
Distributing frame intermediate, C23 

frames, 612 

Divided multiple common battery system, 

390 
■ multiple sub-station circuit, 385 

multiple system, 378 

Double-track trunk defined, 397 
Dougherty, on carbon action in transmit- 
ter, 54 

Drawing in cables, 845 
Drop and jack combined, 197 

and ringer, combined, 206 

for small switch-board, 157 

, self-restoring, Bell, 195 

, Warner tubular, 187 

Drops in strips, 188 

Dry battery, 96 

DuMoncel, variable resistance law, 14 

Edison, carbon transmitter, 15 
Electrolytic cell in line circuit, 301 
Electromagnetic induction, 23 
Electromagnetism, history and principles, 1 
Electro-phonetic telegraph, House's, 11 
Electrostatic induction, 23 
Erdman telephone repeater, 748 
Ericsson receiver, 45 

transmitter, 69 

Exchange, magneto switch-board for small. 

176 
Exchanges in general, 170 
Express system, "A"-board, 252 
, "B"-board, 254 

transfer system, 246 



Fanning cables, 655 
Fault location, S76 
Faraday, discovery of laws of magnetism, 2 
Fessenden, Professor R. A., '"Microscopic 

Telephonic Action," 54 
Ford-Lenfest distributing frame, 617 
Frame of multiple section, 520 
Fuller battery, 90 

Galvanometer, 863 
Galvanometer shunt, 866 
Generator circuits, 153 

, Holtzer-Cabot, 118 

-, Kellogg, 122 

, magnetic calling, 104 

shunts, 116 

, Williams-Abbott, 122 

Gravity battery, 94 

Gray coin collectors, 465 
■ ■ meter, 476 

Professor, Elisha, invention of tele- 
phone, 7 

Gridiron signal, 291 
Grounded line noises, 158 

lines, defects and objections, 158 

Hampton, express transfer system, 246 
Hayes common battery transmission 
system, 270 
Heat coils, 593 
Henry, Joseph, progress of knowledge in 

electromagnetism, 1 
Hibbard distributing frame, 614 

party line, 435 

Holtzer-Cabot generators, 118 

house system, 741 

ringer, 124 

Hook switch, Kellogg, 154 

, Monarch, 156 

, Stromberg-Carlson, 155 

, Warner, 154 

House, Royal E., "electro-phonetic tele- 
graph," 11 
■■ ■ system circuit, 73 1 

Hughes, Professor David B., microphone 
experiments, 17 

Hunnings, Henry, granular carbon trans- 
mitter, 20 

Impedance coil, 559 
Induction coil data, table, 79 

coil, Varley wound, 77 

coil, with transmitter, 21 

coils, circuit for comparative tests, 



- coils for local battery telephones, 73 

- coils, table of various makes, 81 

-, electromagnetic and electrostatic, 

23 
-, local, on lines, elimination of, 16S 



Insulation tests, 



INDEX. 



885 



Intercommunicating system, magneto, 742 

systems, 733 

Intermediate distributing frame, 623 



"ack and drop, combined, 197 
multiple cabling assembly, 672 



strips, 239 

, three conductor, 234 

wiring, 656 

Jacks, answering and multiple, relation, 518 



E 



eith-Lundquist & Ericsons, 693 

Kellogg combined drop and jack, 203 
common battery sub-station circuit, 



common battery transmission sys- 

tem, 272 
— common battery trunk circuit, 366 

four-party line key, 529 

generator, 122 

hook switch, 155 

intermediate distributing frame cir- 

cuits, 626 

jack, 513 

key, 527 

plug, 523 

private branch trunks, 415 

receiver, 41 * 

relays, 539 

ringer, 127 

transmitter, 63 

two-wire, multiple, common battery 

system, 322 

Kelvin, Lord, comment on Bell's telephone, 

10 
Keys, party line, 529 
Key, repeating coil, i93 

, ringing and listening, 190 

, 526 

Lamps, switchboard, types of, 288 
Lay-out of central-office equipment, 651 
Lead burning, 578 
LeClanche battery, 86 
Leich party line, 458 
Lines, grounded, defects and objections, 

158 
, metallic and grounded, intercon- 

tion, 167 

, telephone, 158 

Listening and ringing keys, 190 

Local battery sub-station equipments, 131 

, telephones, induction coils for, 73 

Los Angeles lay-out details, 679 



M 



agnetism, history and principles, 1 
Magneto bell, 109 

, calling apparatus, 104 

, generator, 104 

, multiple switch-board, 225 



Magneto, multiple switch-board, branch ter- 
minal system, 233 
, multiple switch-board, series sys- 
tem, 225 

, switch-board circuits, simple, 180 

Marlin hangers, 817 
Mclntyre sleeve, 799 
Measured service, 460 
Messenger wire, 813 

, table of loads, 814 

, table of strengths, 814 

Meters, 476 
Mica fuses, 592 
Monarch hook switch, 156 
Monitors' equipment, 628 
Morse, Professor S. F. B., electro-magnetic 
telegraph, 4 

, relay, 534 

Multiple cable distribution, 828 

, divided system, 378 

jack cabling assembly, 672 

jack strips, 239 

jack wiring, 656 

jacks, numbering of, 515 

, Kellogg two-wire common battery, 

322 

section frame, 520 

, Stromberg-Carlson two-wire, 327 

switch-board, Bell Exchange in St. 

Louis, 350 

switch-board, common battery, 313 

switch-board, details, 511 

switchboard, magneto, 225 

switchboard, theory of, 219 

Murdock solid receiver, 46 
Murray loop tests, 880 



N 



orth Electric Company's common bat- 
tery multiple, 340 
lamp jack, 516 







bservation circuit, 632 
line, 631 



O'Connell, J. J., originator of incandescent 
lamps for signaling, 278 

Oersted, Professor, discovery of electro- 
magnetism, 1 

Ohm's law, 23 

Operator's equipment details, 532 

Order wire keys, 530 

Packing in transmitters. 71 
Page, Professor, "Pago's effect" in 
magnetism, 4 
Paris switch-board, 242 
Party-line keys, 529 
Party lines classified, 423 
Pay-station coin collectors. 462 
Phelps, George M.. form of transmitter. 17 
Pilot lamp, 521 
Plugs, 523 



886 



INDEX. 



Plug, three conductor, 234 
Pole strips and butt plates, 770 
Poles, table of sizes, 769 

, table of weights, 771 

Poling of receivers, 305 
Pot-heads, 824 
Power board, 556 

;, rear view, 558 

Power plant circuit, 547 
Power plants, 544 
Power table, 566 
Primary batteries, 85 

, method of test, 99 

Protector combined central office, 607 

Protective device, 587 

Protection diagram, 595 

Protector test-plug, 608 

Private branch exchange defined, 396 

service, 396 

, Western Electric cord circuit, 403 

Private branch switch-board types, 420 
Private exchange Kellogg circuits, 415 



R 



eceiver, Ader, 39 

, Bell type, 34 

— cord tips, 50 

, Ericsson, 45 

■ , Kellogg, 41 

, modern construction and design, 

34 

, Murdock solid, 46 

, poling of, 305 

, Stromberg-Carlson, 44 

, watch case, 47 

, Western Telephone Construction 

Company, 43 
Recording operator toll systems, 487 
Reis, Philip, experiments, 5 

, rights as inventor of telephone, 11 

Relays, line and cut-off, 536 

Relay, Morse, 534 

Relays, supervisory, 535 

Repeater telephone, 745 

Repeating coil in cord circuit, 193 

Repeating coil key, 193 

Rheostat, 568 

Ringback key, 192 

Ringer and drop, combined, 206 

Ringer, biased, 129 

coil, Varley, 115 

, Holtzer-Cabot, 124 

, Kellogg, 127 

, Stromberg-Carlson, 125 

, Williams-Abbott, 128 

— — , Yaxley, 126 

Ringing and listening keys, 190 

Ringing circuit, automatic, Kellogg, 372 

, Western Electric, 365 

Ringing machines, 560 

Rorty & Bullard Automatic system, 726 

Ross, on carbon action in transmitter, 54 



Sabin, express transfer system, 246 
Sag table, 800 
Saint Louis Bell switch-board, 350 
Saw-tooth arresters, 5b8 
Scribner coin collector, 466 

common battery system, 296 

meter system, 482 

Seasoning poles, 768 

Selector circuit, Automatic Electric Com- 
pany, 709 

Selector switch, 696 

Self-soldering heat coil, 609 

Series local battery sub-station equipment, 
134 

Service observation circuits, 632 

Service observation line, 631 

Shunts, for magneto generator, 116 

Signaling in common battery systems, 277 

Single-track trunk defined, 397 

Solid-back transmitter, 60 

Splicing cables, 820 

Springjacks, 512 

Springjack and plug for small switch- 
board, 178 

Sterling common battery multiple, 336 

Stone common battery transmission sys- 
tem, 268 

telephone repeater, 749 

Storage batteries, 572 

Stromberg-Carlson common battery sub- 
station circuit, 308 

common battery trunk circuit, 373 

hook switch, 155 

jack, 514 

lamp and answering jacks, 517 

receiver, 44 

relays, 542 

ringer, 125 

three-wire multiple, 333 

transmitter, 65 

two-wire common battery multiple, 

327 
Stroud coin collectors, 471 

meter, 478 

Strowger, A. B., 692 

Sturgeon, William, progress of knowledge 

in electromagnetism, 1 
Sub-station automatic equipment, 702 

, common battery, Western Electric 

circuit, 307 
, divided multiple, 385 

equipment, common battery, 304 

equipments, local battery, 131 

local battery circuits, 138 

protection, 602 

Supervisory lamp, 521 
Sutton transmitter, 68 

Switch-board circuits, simple magneto, 180 
, common battery, for small ex- 
changes, 293 
, common battery, signaling in, 277 



INDEX. 



887 



Switch-board, common battery, transmis- 
sion systems, 265 

frame, modern, 520 

keys, 526 

lamps, types of, 288 

, magneto, for small exchanges, 176 

, magneto multiple, 225 

, multiple, common battery, 313 

, multiple, theory of, 219 

, rear view, 485 

, small, standard types, 209 

transfer systems, 245 

, transfer system, Western Tele- 
phone Construction Company, 
256 

Table of breaking weights of copper 
wire, 763 

cable capacity, 810 

copper wire, 761 

' cross-arm sizes, 773 

differences of wire gauges, 757 

galvanized iron wire, 760 

• induction coil data, 79 

messenger wire data, 814 

■ pole sizes, 769 

pole weights, 771 

— resistance of copper wire, 764 

rubber-covered cable, 806 

sag in spans, 800 

— specific inductive capacities, 29 

of various makes of induction coils, 

81 

weights of cable, 812 

Telegraph, House's "electro-phonetic," 11 

, Morse's electro-magnetic, 4 

Telephone, invention of, 7 

■ lines, 158 

, magneto, history and principles^ 1 

■ repeater, 745 

Tests for induction coils, comparative, cir- 
cuit for, 83 
•Test plug protector, 608 
-Test, primary batteries, method of, 99 
Testing, 851 

by wire chief, 635 

Thermopile for transmitter supply, 276 
Thompson meter system, 481 
Thompson & Robes, party line, 440 
Thomson, Sir William, first English ex- 
periments, 10 

Three-conductor jack and plug, 234 

Tips, receiver cord, 50 

Toll switch-board, type, 509 

systems, 485 

Toll trunk line, Western Electric, 490 

Transfer systems, 245 

Transfer system, Western Telephone Con- 
struction Company, 256 

Transmission systems in common battery 
exchanges, 265 



Transmitter, Ahearn, 71 

, Berliner, 14 

, Berliner Universal, 59 

, Blake, 55 

, Clamond, 58 

— , Colvin, 67 

, Crossley, 57 

, Edison carbon, 15 

, Ericsson, 69 

, Hughes' experiments, 17 

, Hunning's granular carbon, 20 

, Kellogg, 63 

, liquid, 13 

, modern construction and design, 

53 

, packing in, 71 

, Phelps form, 17 

, Stromberg-Carlson, 65 

, Sutton, 68 

, Turnbull, 57 

, variable resistance, history and 

principles, 13 
, Western Telephone Construction 

Company, 70 

, White or solid-back, 60 

Transposition, 163 



Trunking system between common battery 
offices, 354 

Trunks, single-track and double-track de- 
fined, 397 

Tubular fuses, 603 

Turnbull transmitter, 57 

Turning section view, 669 

Two-wire multiple, Kellogg, 322 

, Stromberg-Carlson, 327 



u 



nderground cable construction, 835 



'arley induction coil, 77 
loop tests, 877 



ringer coil, 115 

Visual signals, types of, 291 



w 



arner hook switch, 154 
tubular drop, 187 



Watch case receiver, 47 

Western combined drop and jack, 197 

Western Electric central office meter. 4S4 

common battery circuit, 307 

common battery multiple switch- 

board circuits, 314 

common battery trunk circuit, 35S 

four party line key. 529 

intermediate distributing frame cir- 
cuits. 625 

— ; jack, 512 

keys. 526 

« — ttt relays. 535 



888 



IXDEX. 



Western Telephone Construction 

pany receiver, 43 
, transfer system, 256 

transmitter, 70 

Wheatstone Bridge, 855 

White, or solid-back, transmitter, 60 
Williams-Abbot generator, 122 

ringer, 128 



Com- Wire chief's equipment, 635 

testing circuit, 643 

testing trunk, 636 

Wire for telephone use, 752 

Wiring of central-office equipment, 651 

"\7axley ringer, 126 



THE END. 






UAg?9 



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LIBRARY OF CONGRESS 



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